I kinda go by Carl and Sophia's apparent ages from first issue to current. Although, now that I think about it, that might put us closer to a 4 year span in time.
Carl's gotta be about 16 by now, and he was definitely no older than like 12 at the outset.
He's certainly old enough to know how to beat a guy with a shovel upside the head
Oh yeah -- and Maggie's rugrat looks no more than about 2.
EDIT: Re-reading back just before Savior arc and Carl is "9... I think", with the implication he'd be about 9 1/2 (right before the eye loss), and that is like 1 1/2 years into the zombie apocalypse. So, if hes' even only 14ish post Savior arc, I think that makes it around 4+ years have elapsed since the beginning of the story.
EDIT #2: Rick comments in the next bound edition that it had been about two years since people stopped canning foods -- implying that, yes, it was between 1 1/2 to 2 years at that point since the end of the world, and given some (ahem) new situations with Carl in the latest issue, Carl is between 14 and 16 now.
So, I'd say it's 5 years max into the time line from the day the dead first walked is where Kirkman has us as of Feb 2015 in the comic.
On the TV show, that would put them right about at the 1 1/2 to 2 year mark, I think.