According to this site, they were expecting 3.3 billion people flying during 2014. https://www.iata.org/pressroom/pr/Pages ... 30-01.aspx
Using obj's calculations which seem pretty reasonable, you get 9 million/day. With an average flight time of 1.5 hours, that would be 9m/16 = 565,000. So for there to be 1 million at any given time, the average flight time would have to be 2 hours and 40 minutes. Which world wide may very well be.
The hard part would be peaks and troughs which are local. Peak in the US is trough time in Asia and vice versa. So it probably evens out over a 24 hour period.