When I was 6 months pregnant (I'm 8 now, 35 weeks) I remember getting regular back and leg massages and I was facing down. It depends if you feel comfortable that way or not.
I'm 30 weeks and still able to sleep on mine.
I'm going on 18 weeks and still sleep on mine. Whenever it stops feeling comfortable basically.
About 15 weeks I now cram the majority of the duvet between my legs x