Many have questioned the ability of the Still-Alives to have survived the zombie apocalypse, considering the stupid decisions they make, like the well debacle (2x04 - "Cherokee Rose"). But it's possible that they actually haven't been alive for very long.
How many days have passed from the start of the show to the events of episode 2x05 - "Chupacabra"?
After taking a random guess, I figured I'd do some more research and find out more details. However, exact details on length of time are hard to come by. So, some of this is still guesswork.
First, Rick's coma: The Walking Dead Wiki says that Rick was "Rick was wounded in a shoot-out, and was put into a coma for about a month.". Meanwhile, AMC's site about The Walking Dead says "Waking up in an empty hospital after weeks in a coma...", so most of the media says that Rick was in a coma for at least a month. It's possible that a week or two of that time was spent when the hospital was still operating, though. As for the starvation/dehydration issue, one can assume that Rick was hooked up to an IV for part of that time, and it could just be writer's prerogative (also, being in a coma probably reduces the need for food/water)
Second, in the episode Wildfire, Dr. Jenner says "It's day 194 since Wildfire was declared,", which would actually be about 6 and a half months from the "first outbreak". Now, it's hard to say just when wildfire was declared, in terms of 'society is completely overrun' or 'the very first instance happened, and we managed to contain it'. If we assume the former, that means the events of the first season lasted 5 months. That doesn't really match up with what we see on the screen (we'll get to that later...), so we have to assume it's the latter. But, it's been about 6 and a half months since the first instance of a zombie, which was likely contained and news about the incident suppressed.
Finally, the actual events of the show. This is mostly going to be guesswork, due to not having time to watch the entire show from start to finish at the moment. But, Rick wakes up, and goes to out (meeting with Morgan and Duane). They spend a night in a house, and go to the police station to get guns and supplies. Rick heads to Atlanta, taking an unknown time to get there (King Country, Georgia is a fictional location). Given that Rick runs out of gas on his way to Atlanta (and assuming that he filled the tank up when he left), we'll assume that it's at least 240 miles away (a 4 hour drive at 60 mph). This puts us at day 2 when Rick gets to Atlanta. The events in Atlanta and the camp seem to take about 3-4 days (1 day for "Guts", 1 day for "Tell It to the Frogs", 1 or 2 for "Vatos"). Going with a slight margin of error in case of skipping a day, we'll say that it's 5 days, giving us 7 days since Rick woke up.
The CDC is actually located in a neighbourhood of Atlanta, which makes the actual travel time fairly short. The events of the episode TS-19 seem to take 3 days (getting there, feast at night, one day of rest, one day when everything shuts down). This puts us at 9-10 days since Rick woke up, and the end of season 1
Season 2 starts with the group leaving the CDC, and heading for... somewhere. The events of the season 2 episode "What Lies Ahead" seems to take up 2 days, though we'll assume the beginning of the day here is the same day as the end of season 1. "Bloodletting" adds at least one more day, and ends at night, and "Save the Last One" takes place almost entirely during that night.
As of "Save the Last One", we're up to somewhere between 12-13 days since Rick woke up. At the moment, that's the last episode I've seen, and I don't want to spoil any of the plot for myself beyond that. I will try to update this answer as I see more episodes, though.
It's hard to tell exactly how much time has passed from the outbreak to the start of the show. But according to Jenner (1x05):
It's day 194 since Wildfire was declared and 63 days since the disease abruptly went global.
It doesn't help that he logs this in an unspecified point in time. It's difficult to imagine Rick survived for over 2 months by himself in the hospital after it's been overrun. Not much adds up here.
Focusing on the events of the show:
1x01 (Days Gone Bye): Rick wakes up, wanders around the town and is found by Morgan. They spend the night at Morgan's house (Day 1 from Rick waking up).
The next day he heads to Atlanta. During the car chase in the beginning, the radio dispatchers mention the car is at "Route 18, two miles west of I-85." That would place them in West Point, Georgia, which is is 1h30min away from Atlanta. Regardless, I think it's safe to assume Rick gets to Atlanta in the same day. (Day 2)
1x02 (Guts): Rick is found by the Survivors, and they try to escape from the building. (still Day 2)
1x03 (Tell It to the Frogs): The Survivors escape from Atlanta and Rick reunites with his family. (still Day 2)
1x04 (Vatos): Rick et al. head back to Atlanta to save Merle and get their guns back. They return at nightfall to find the camp being attacked by zombies (Day 3).
1x05 (Wildfire): This happens exactly the day after the attack, according to Rick's description in the beginning of the episode. They head to the CDC and reach it by dawn. (Day 4).
1x06 (TS-19): It's presumably night when the Survivors dine. (still Day 4) The very next morning (evidenced by the fact that some people in the group are hung over, and from what's said during breakfast), the Survivors learn how everything went down.
They also notice the countdown. Eventually, they escape the CDC. (Day 5).
So the events of the entire first season transpire in just five days according to my estimates.
I will watch the second season and update this answer.
Rick appears to have been receiving a maintenance IV of possibly 0.9 normal saline or Lactated Ringers to keep him hydrated due to fluid loss or possibly a hypertonic solution to decrease intracranial pressure if his coma resulted from respiratory failure. His medical condition would have been stablized prior to the ZA because he is not connected to a vent or any critical drips. The bag's volume appears to be 1000 ml and the rate of administration would be generally 50ml/hr for a man of Rick's size. This bag would take approximately 20 hours to complete before hanging a new bag.
He is not receiving tube feeding. He has no peg tube and no nasogastric tube to receive liquid calories, protein, fats and carbs. There is no feeding pump connected to the IV pole. He has no secondary IV bag or antibiotic hanging with the primary bag in the event of sepsis from his gunshot wound.
Also, Rick's coma probably did not result directly from the GSW. Comas unrelated to head trauma can be caused by metabolic abnormalities or they can be medically induced in patients who have suffered respiratory failure and require mechanical ventilation. An induced coma would be done by a Propofol drip through a central IV line. When these drips are stopped it only takes minutes for the patient to awaken.
The scene with Shane standing over Rick with the flowers makes it hard to determine how Rick is ventilated but there is no IV sedative hanging on the pole behind him and later Rick abruptly wakes up with only a nasal cannula in his nose. In a real world scenario Rick needs to wake up very soon to survive. One to two days after the ancillary staff have left the hospital.
After the episode "Save the Last One":
The next season will most likely take place on the 24th day after he has woken up from the coma.
Day one: Rick wakes up, gets found by Morgan and sleeps in the house he has set up camp in.
Day two: Starts with Rick hitting the walker with the baseball bat. Ends with him sleeping with Lori at the camp in episode 3.
Day three: Starts with Rick and co. heading to Atlanta to rescue Merle and the nag of guns. Ends with the camp being overrun and Amy dying.
Day four: Starts with the group burying/burning bodies from the camp attack. Ends with the group sleeping in the CDC after their meal and Shane getting scratched by Lori
Day five: Near the beginning, Rick comments on Shane's scratch mark he got from Lori. This must mean it happened the the following morning from arriving at the CDC, as Rick would've noticed the scratch earlier. Day ends with the CDC blowing up.
Day six: Not seen as Rick mentions in S2E1 "we lost somebody day before last".
Day seven: Starts with group heading to that other camp. Ends with Shane shaving his head at the end of S2E3.
Day eight: Starts with Otis' funeral. You can tell because the Atlanta group show up and it makes sense in this situation to have the funeral the day after he died. Day ends with Lori finding out she's pregnant as Glen got the test the same day he got used as bait, and it's unlikely Lori would have waited another day to use the test.
Day 8: Rick actually comments in S2E5 on how many days it's been since Sophia went missing, but I can't remember what he says. This day ends with Daryl recovering after being shot.
Day 9: This day may not have been seen by us as well, as Daryl is now in his tent and may have needed more bed rest after his incident. Let's say it is the next day -- it ends with Lori telling Rick her secrets.
Day 10: this is s2E7. Day starts with Glen telling of the walkers in the barn. Ends with Lori back at the farm after her incident and awaiting Ricks return from finding Herschel
Day 11: Starts with Rick and co. returning with Randall at the end of S2E9. There is a 10 day gap between ep 9 and 10 to allow Randall to heal after his wound.
Day 21: This day took place during S2E10 (which I actually have an original, signed script for :D)
Day 22: Starts with Daryl and Randall in talking in the shed. Ends with Dale's death.
Day 23: Starts with Dale's funeral. Ends during the farm attack, as it is morning when the group meet on the highway.
Day 24: ends with the view of the Prison.
What a busy month!
According to Axel in S3E02 (Sick), they had been locked in the cafeteria for 294 days (9.6 months). It's unknown if they were locked in at the initial onset of the outbreak or if it was a day or two later.
Rick left the Hospital on ZA+60 (after several weeks (21 days is several weeks), though there would have been a functioning infrastructure even during the initial outbreak. Once panic, and breakdowns occurred, the Hospital would go automatically to generator (usually a 2 week supply - could be more if chaos and the hospital was abandoned - less resources used) - Shane visits Rick 2 weeks into his coma, Shane comes again, blocked Rick's (ZA+46) room when the military showed up and started shooting. So 1 week after Shane's attempt to block his door he walked out of the hospital in ~West Point, GA (90 miles from Atlanta).
Rick met up with Lori on ZA+62; on ZA+70 Lori asks for a pregnancy test. ZA+84 is the last episode in Season 2.
Season 3, episode 1 Lori is due any day (8 months have passed) - so if Rick is the baby's daddy (ZA+/-329), if Shane is the baby's daddy - (less than 8 months have passed (why Rick is not too happy - don't need to be math genius here)) ZA-322 (before Rick's arrival at the quarry and whoopie.)
Now that is all in the sense that Lori is going full term and is not pre-mature.
I would say his iv bag was still in him so I think he could have lasted about a week. However, in the flashback it shows Shane trying to rescue Rick, but the power goes out. But the outbreak seems to have already gone global and the zombies were already in huge numbers.
Since they live in a somewhat remote area it may have taken time to get there so the hospital could have kept him alive as the infection continued to ravage the state so he could have been kept alive for months as the infection started, then woke up shortly after the power went out.
This first section has been pilfered from my own answer to another question:
Rick's group got to the farm maybe a month and a half after the outbreak started - maybe less - and left after only a few weeks (Lori wasn't visibly pregnant when they left).
Season 1 took place over the course of a few days (not including the shooting that put Rick in the hospital) - fewer days passed than there were episodes in the season; by my count, 5 days total, vs. 6 episodes.
Day 1: Rick wakes up and gets to Morgan's house, sleeps there. Day 2: Rick leaves Morgan at the police station, goes to Atlanta, meets Glenn and the others, and goes to their camp, where he finds his family; they all go to sleep. Day 3: Rick, Daryl, and T-Dog go to Atlanta to look for Merle, meet the Vatos, go back to camp after dark, and find it under attack; Amy and Jim are bitten, no one sleeps. Day 4: Amy and Jim die, the group goes to the CDC, gets drunk, then showers, Shane tries to rape Lori, everyone goes to sleep. Day 5: The group escapes the CDC, Jacqui dies in the explosion, the group spends the night camping somewhere near Atlanta (the season 2 debut begins with Rick on a rooftop overlooking Atlanta the next morning, talking on his walkie-talkie).
Season 2 began the day after the escape from the CDC, and ended about a month later, maybe a bit less, maybe a bit more, but they were on the farm for no less than 2-3 weeks, no more than 6 weeks - enough time for Lori to realize she was preggers, but not enough time for Shane's hair to grow back (he shaved it off the first night they were there, which was two nights after they left the CDC).
And since Rick would have died of dehydration if his coma lasted more than a week or two after the hospital was abandoned, and the hospital was abandoned shortly after Rick got there (things went bad VERY quickly, and hospitals were the first places to fall because that is where everyone who was bitten went), Rick couldn't have been in the hospital for very long.
When Rick woke up, he had a bit of stubble, but not much - I'd guess about 2 weeks' worth by average beard-growth rates - and the lights and water were still on; so he can't have been in the hospital for more than a week or two after it fell, maybe 3 weeks total after he was shot. Finally, Shane and Lori only hooked up after the city fell, probably after they got to the quarry campsite; she probably wouldn't know she was pregnant for a month or so after conception (she'd notice she missed her period before morning sickness started happening). So they left the farm no more than 8 weeks after the outbreak began, possibly as little as 5 weeks, probably closer to 6 weeks.
By the time they reached the prison, Lori was very pregnant, and they had spent the entire winter on the road (they said so); it seemed very warm already, so it was probably mid-late spring; on the farm it seemed warm, but not terribly hot, and they were worried about it getting cold soon (the subject came up late in season 2). This suggests that they left the farm in early/mid fall (leaves were still on the trees and still green). So between the end of season 2 and the beginning of season 3, at least 4 months passed, possibly as much as 6 months. That is 4-6 months of survivors dying and coming back as zombies. 4-6 months of zombies leaving the abandoned cities and spreading slowly across the countryside. 4-6 months of zombies clumping into groups, groups merging into herds, and herds roaming around, sometimes aimlessly, sometimes chasing prey.
Chupacabra happens shortly after the group arrives on the farm, because the need to search for Sophia is still the top priority, and Carl's injury is still just beginning to heal.
However, this contradicts what Dr. Jenner at the CDC said in episode 5 of season 1:
It's day 194 since Wildfire was declared and 63 days since the disease abruptly went global.
I see no way to reconcile Jenner's statement with the events on the show, unless Rick was in the hospital for much longer than I believe. Rick was clearly surprised by what he found when he woke up, so he obviously didn't know anything about the outbreak when he was shot and went into a coma. As a first responder, he would presumably have been told at least something about the outbreak, had it been in progress at the time of his shooting.
So if Jenner is correct about the timeline, and if the show is telling us that he made this statement the day that the group arrived at the CDC, then Chupacabra takes place about 200 days after "wildfire was declared" (whatever that means) and about 70 days after "the disease abruptly went global".
If my estimation is correct, on the other hand, then Chupacabra takes place about six or seven weeks after the outbreak began.
The TWD wiki's timeline says that Chupacabra takes place at least 69 days after the outbreak began.
Day +69:
The next day Lori wakes up and talks to Carol about using the farmhouse kitchen. Meanwhile the group is still searching for Sophia and Jimmy joins them. Shane decides that he will begin gun training the next day. (Chupacabra)
Shane questions Rick’s decision to keep searching for the girl. When they return Rick and Lori talk about continuing the search. Beth calls for Rick to talk to Hershel. Hershel is not happy about Jimmy joining the group and Daryl taking a horse. He and Rick agree to control their own people.(Chupacabra)
Daryl finds Sophia's doll in a riverbed. He continues to search but his horse throws him and he falls on one of his arrows. He has a vision of Merle and awakens to find a walker biting at his work boots. He dispatches that walker and another and then climbs out of the ravine. (Chupacabra)
Glenn and Dale talk about the women and Glenn admits he had sex with Maggie. Andrea thinks she sees a walker emerge from the woods. Rick and co. approach the person to discover it is Daryl. Andrea blinded by the sun shoots Daryl in the temple. Later Hershel tends to Daryl’s wounds as Dale comforts Andrea. At the supper table there is awkward silence as Maggie slips Glenn a note. Glenn returns the note. (Chupacabra)
That evening Maggie checks the note and realizes Glenn is in the barn. She runs to the barn to find Glenn has discovered a number of walkers. She tells him that "[he wasn't] supposed to see this" as the walkers push at the barn door. (Chupacabra)
For the role of Judith, the producers have been swapping out babies on a regular basis to make sure the character ages at the proper rate. So far, 16 babies have played Judith.
In most of season one, all of season two, and part of season three, Judith had been conceived, but not born. She was conceived either by Shane and Lori while Rick was in his coma, or by Rick and Lori immediately after he found Lori and Carl.
By season six, the role had fallen to Chloe and Sophia Garcia-Frizzi. Judging by the photo below, the twins appear to be roughly 18 months old:
Since the producers have chosen actresses who are the right age for the part, we can use this to work out how long ago the inevitable zombie apocalypse began.
Between power going down in the hospital and Rick depleting all supplies in his room was just a few days. Remember there was some time between the outbreak and closure of the hospital. So basically if he was fed intravenously in the hospital and then wasn't for only a few days, then he could survive. Weak, disoriented but... it's possible.
28 days later, too)