I know we don't have any official confirmation regarding the time passed since the outbreak, but here is my theory and  correct me if I am wrong.

Lori got pregnant either by Rick or Shane. Rick met them one month after the outbreak, so this is ten months.

Now Maggie is pregnant and is about to give birth, so this gives us another 9 months. 

The only question is, how much time has passed between Lori giving birth and Maggie staying pregnant? It's been a while since I last read the coomics, so if you coud help me that would be great. Until now, we have one year and 7 months, but I can't see another 1,5 years between these two events. So I 'd say it's been two years tops

What do you think?