(STILL EDITING!)
I made a rough estimate about how long has passed since the outbreak on TWD's TV Show based on the roughly estimated timeline on the TWD's Wiki (linked at the bottom of this post) and my memory from recently re-watching the whole show. Just remember we don't have exact dates, actor ages/appearances don't necessarily correspond with the passage of intended time and the passage of time is deliberately left ambiguous in both the TV show and the comic, so hard to estimate, but here you go:
2010 = Outbreak Began
Between Outbreak & Rick awakening = 3 months
Season 1 = 5 days
Between Season 1 & 2 = 1 day
Season 2 = 17 days
Between Season 2 & 3 = 7/8 months (an entire winter)
Season 3 = Occurs in a span of less than 2 months, 21 days
Between Season 3 & 4 = 6 months
Season 4 = 11 days
Between Season 4 & 5 = No time
Season 5 = 36 days
Between Season 5 & 6 = A number of days, 5 days
Season 6 = 50 days
Between Season 6 & 7 = Some minutes
Season 7 = 12 days
Between Season 7 & 8 = 11 days
Season 8 = 6 days
Between Season 8 & 9 = 1 ½ years
Season 9, Episode 1 - Season 9, Episode 5; Rick's disappearance = 37 days
Season 9, Episode 5; Rick's disappearance - Magna's group's appearance = 6 years
Season 9, Episode 5; Magna's group's appearance - Season 9, Episode 16 = 134 days
Between Season 9 & 10 = 159 days
Season 10 = 149 days
Between Season 10 & 11 = 1 day
Season 11 = 6 days so far (season still ongoing)
Outbreak - Season 11 = 7 ½ years, 18/19 months, 512 days and some minutes = 10 years, 4/5 months, 3 weeks, 6 days and some minutes since the beginning of the outbreak
7 ½ years, 18/19 months =
7 years and 6 months
1 year and 6/7 months
9 years/9 years and 1 month
512 days = 1 year, 4 months, 3 weeks and 6 days
9 years/9 years and 1 month + 1 year, 4 months, 3 weeks, 6 days and some minutes =
10 years
4/5 months
3 weeks
6 days
Some minutes
10 years, 4/5 months, 3 weeks, 6 days and some minutes
√√√√By season 11 it has been 10 years, 4/5 months, 3 weeks, 6 days and some minutes since the beginning of the outbreak so far.√√√√
https://walkingdead.fandom.com/wiki/TV_Series_Timeline
The outbreak was confirmed to begin in 2010 by Date of Death; the thirteenth episode of the second season of AMC's Fear The Walking Dead.
https://walkingdead.fandom.com/wiki/Date_of_Death
The apocalypse most likely started in the winter of 2010. I think this due to background cues on Fear The Walking Dead. At the school shown in the show, students are wearing hoodies and light sweaters, but there is no indication of an upcoming holiday in any of the classrooms or hallways. There aren’t any Halloween decorations to allude to the month of October or any Christmas decorations to allude to late November/December or any Valentine's Day decoration/dance posters. This indicates they’re in the celebratory ‘dead zone’ of late February/early March when it's still cold outside, but there aren’t any major holidays in sight. This theory is solidified by the handful of bare deciduous trees peppering the park. Also, the absence of new life, showing it isn’t spring yet.
Winter of 2010 also aligns nicely with the seasons as they unfold on ‘The Walking Dead.’ One of Madison's students mentions the outbreaks have occurred in five states by the opening of ‘Fear the Walking Dead.’ Most people still seem unaware that any of the illnesses are related, much less cognizant of the fact the dead are reanimating (with hints the government is keeping the public in the dark on purpose to keep panic at bay).
Whatever the cause, the Walkers seem to be spreading erratically, but with increasing speed, making it completely feasible for Los Angeles to be infected in the winter and Rick to awaken post-coma in the Georgian spring (based on vegetation), to a completely devastated world tipped over the edge by a Walker event horizon. Let's say it's late February in ‘Fear The Walking Dead’ and Rick awakens in late May. That’s three months from fully a functional society to a dystopian wasteland.
https://uproxx.com/hitfix/fear-the-walking-dead-quietly-confirmed-what-year-the-zombie-outbreak-started/
Carl was born in April 1998 (starting the show off as 12), so by my calculations, he should’ve been about 15 if he was still alive. However, he was confirmed to be 14 in season 4, but my timeline would’ve made him 13, so I guess that’s not too different. Although, he clearly looked even older when he died due to Chandler aging, most people estimate he was about 15/16 (S8E9 Honor) at that point, whereas my timeline would still make him 13 (almost 14). I’m not really sure how to reconcile that because that’s about 1/2 years and 4/5 months missing...
Judith was about 3 when Carl died, so that makes them at least 12/13 years apart, so he must've been 12/13 when she was born. She's confirmed to be 10 in Season 10.
(STILL EDITING!)