Season 1 and the first half of Season 2 took place within one month. After "Nebraska", it was a week by week basis until it lead into winter (Lori performed her pregnancy test during the first half of Season 2, which was still during the first month of the zombie apocalypse. That means she was at least a couple of weeks and a couple of days pregnant). By the time Season 3 occurs, she's already 9 months pregnant. So, at minimum, it's been 9 months since the start of the zombie apocalypse, give or take 6 days (remember, Rick had been in the hospital at least a week before the outbreak became really noticeable and it had been at most three days after Shane left him in the hospital in the flashback before he woke up, as the IV bag that kept him hydrated only lasts for at least for a 24 hour period, 72 hours at most depending on how slow the drip is set. This is an important factor, as a human being can only last 3 days without water and a couple of weeks without food, depending on the human being's health and body type. And before you cite
Aron Ralston and his five days of being trapped in a canyon due to his arm being pinned to a wall due to a heavy boulder, keep in mind that he was an experienced climber and knew how to ration his water supply for a three day period, which after it was gone he began to drink his own urine).
For the comics itself, it's been over two years where they are current at for that particular version of the story.