I don't have time to read through all the replies and reply myself, so I apologize if I say something other have already said. This is what we did with Nathan...

Nathan was in an infant seat until he was 11 months (give or take a little). Then we moved him to his Britax seat, rear-facing. We kept him rear-facing until he was 20 months old, then we turned him around.

As I skimmed the other replies I noticed someone mentioned the infant seats that can come out of the base. That's what we used with Nathan, but we never used the carseat out of the car. I think we maybe one or twice, because of bad weather, brought the carseat into the house to strap him into the seat so we didn't have to do it out in the rain with our backs getting wet. It's just a personal preference, but we never used that feature. If/when we have baby #2 I may not even use the base.