Our hospy physios said 16 weeks. But I was told this at 32 weeks I kind of vaguely knew about it, but went with the "if I'm comfortable" thing. Then at 32 weeks my DH knew about it too, and wouldn't let me do it anymore I was comfortable on my back right up to 40 weeks. Huh.

For exercise though, I'd cruise around on the safe side of things.