The question says it all, how much time as passed from the start of the Zombie outbreak to the start of Season 3?
Using Laurie's pregnancy you can say "around 9 months" but we don't know if she became pregnant before or after the fall, and if after how long after.
Is there a canonical answer? Possibly something from the comic books?
I've no real reason for asking other than me & the missus are curious.