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.