I'd say six months for Stewie.
Here he is @ 16 weeks (first day I brought him home):
http://img.photobucket.com/albums/v5...le/stewie4.jpg
here is is @ about 6 months:
http://img.photobucket.com/albums/v5...e/6421119b.jpg
I feel like I missed out since I didn't get him @ 12 weeks. I mean, some people get their babies as young as 8 weeks. Stewie was twice that age when I got him