I think you're meant to keep them rear facing until at least 9 months, because their necks just aren't strong enough to forward face, even if you aren't keeping them rear facing for longer.
Yes Spotty, it's when their ears reach the top of the seat or when they reach the maximum weight, ideally.
I'm looking for A to stay in his seat for as long as possible, and at least until he's 1, so that I don't have to move DD into a booster too soon. At least his slower growth means that should be possible!
Utopian an extended rear facing seat is safest, but they are extortionate. Especially when the insurance company doesn't pay to replace it after an accident < bitter emoticon >. Have a look on the car seat board in products, there's loads of info and suggestions on there. Take Which reviews with a pinch of salt though. They focus more on how easily the cover comes off than safety, and assume everyone is an idiot who can't follow basic instructions.