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I studied abroad in Africa this past semester and that required a 16 hour flight both ways, air time only. On the way back, I was stuck in that ash cloud mess and got stuck for a week (I had $70 to last me a week in the city the airport was in, including food and hotel; oddly enough, that week was the best part of my whole trip), and when I finally did get to go home I wound up having to spend the night in London Heathrow (18 hours) before flying back to Chicago. I did get a whole row of seats to myself, though, which was nice since the flight I was on was reportedly for stranded folks primarily. :P
I can't sleep when I fly no matter how hard I try. 4 tylenol PMs, wine, and just 3 hours of sleep on a airport bench before taking off still couldn't do it for me.
I'd say 20 hours (air time) is my limit. I'd go bat**** unless I was able to sleep, or had a 3+ hour layover at the midpoint of the trip.