For those of you who use the Britax Marathon convertible car seats, when did your LO get too big to be rear-facing?

I read some stuff about babies outgrowing the rear-facing before parents were ready to switch them and am curious. We have this car seat for our boys and at 1 year, I feel like their legs are running out of room! The seats are currently reclined in the middle position. I tried to move them to be upright but it seemed like they had even less space then!