It obviously depends on the law, but in the US it's rear facing up to 1 year AND 20lbs. Here in Europe, where I now live, it depends on the seat type, but type I needs to be rear facing. My kids have been rear facing up to 15 months (26lbs) in US Britax seats.
ETA: Britax will go rear facing up to 33lbs, like most US seats (33-35lbs). In most European countries only the type I seats (infant carriers) will go rear facing (which I personally think is a shame and wrong), and most children will get out of those before age 1 (because of height, they go up to 13kg).