They say as long as 9 months, but judging by the length of DD, she won't be in it too much longer. Her feet are at the end of the seat now, although like LA says, I'd prefer her to be in rear facing seat as long as possible!
My DS is big and is still in his at 6 months. You can remove a pad at the bottom and the straps can be moved up. I bought the footmuff as well and its great!
DD was 6mths, very tall for her age- after which we moved her to the Maxi Cosi Tobi..
Just moved our DS & he's a big babs. He was 9lb 13oz's & v.long.
It's only when their heads come over the top of the seat that you need to move them.