Boeing have built aircraft for a couple of years now, so I expect they have a clue on how long they last before problems set in.
I expect and the manufacturer that the "average" flight of a B767 to be much greater than 2 hours. So if the manufacture says 50,000 cycles (flights) is the limit, then we are talking way over 100,000 flight hours. But they are not concerned on hours - why are you?