If you're prone to taking left-hand sweepers at high RPM's and aren't a fan of running a little extra oil in the engine (or slapping on a DMD for crank harmonic issues), not terribly long at all it seems.
Water pumps seem to fail around 50-70K, though mine looked perfect when I replaced it (at 59K). If you don't catch the temps rising on your engine, it will cook itself prematurely given this failure.
If you are aware of these issues and perform routine maintenance on the car, it should last you quite a long time. I seem to recall more than just a few people on this board that have their 2.5L Duratecs up in the 200K mile range.
I've got 76K miles on my 2.5L and it's running strong and sees redline more than a couple times a week.
If you are looking for a mathematical average, I can't help you as I don't think anyone has been keeping track of mileage on the Duratecs on this board.
