http://walkingdead.wikia.com/wiki/The_Walking_Dead_TV_Show_Timeline
According to the timeline it’s been less than fifty days since the prison fell and Herscel lost his head. That was 3 seasons ago
Yes, it’s easy to forget that the characters have not had a lot of time to adjust to all of the changes and events in their lives.
I gave up on timeline when one day Judith is born and then all of sudden she looks two years old.
Interesting, according to that analysis,
a year and a half has passed since the first episode.