I just made a 12 hour flight and I was fine. Be sure to get-up and stretch, maybe get some pressure socks to combat dvt risk and also make sure you get a "fit to fly" letter from your GP as most airlines will not let you board without this after 28weeks.
My friend flew 8 hours at 33-34 weeks and was ok. Just stay hydrated and get up and walk.
When I asked my doctor about flying she said to make sure and get up and stretch your legs at least every 2 hours. Goddess luck!