I think about 15 hours. I cannot sleep on planes. I have traveled for more than two days straight before, from 12 hour plane rides, to layovers, to six hour connecting flights, to another lay over, to another few hours on the plane. I can be practically high from exhaustion, and I just cannot fall asleep on a plane for the life of me. So after travelling like that, I just pass out for the better part of a full day.