Quickest going out seems to be 14h 30m including stopover time (Gatwick -Philadelphia-Cancun ) but the shortest total flight time is just under 11hrs ( change at Atlanta )
Hiya
I've flown to Cancun from Gatwick 3 times in the past and each time, the flight time has varied quite a bit. First time I flew, it took nearly 14 hours there, approx 13 back. The last couple of times I've flown, it's been closer to 11 hours there, 10 back.
Well worth it though - just stock up on good books & tunes for the flight!! :)