It completely depends on the baby and the limits on the seat. Some seats have higher limits than others. I know someone who had a petite daughter in her Chicco infant seat for around a year! She did not outgrow it lengthwise or weight wise.
My DS was done with his at 6 months. It had a 29 inches height limit and he was already 28 inches long by then. A lot of babies outgrow the height limit first.
I should have clarified my car seat goes to 22 lbs my daughter weighed 11 lbs at 2 months so I think she is on the smaller end.
Thanks for your replies...
Ya it does seem like it ranges from 6 months to a year from the moms I have talked to.
We just switched to a convertible (myride 65) about 2 weeks ago. DS still fits in his infant seat (snugride 32) but it was just too heavy for me to carry around anymore and I would just take him out of it anyway when we'd stop. H still uses it sometimes since the convertible is in my car. We are going to get an inexpensive cosco for our upcoming trip to FL and to keep as a spare in H's truck.
Like PP-it depends on the size of your LO.