On the Epic Ride, I changed my PR3s at different times. I put on a new rear when the old one had 13,560 on it. It would have gone another 2-3000 easily, but that would have put me in Canada while looking for a new tire and a dealer to install it.
The front got replaced at Twisted Throttle when the old one had 18,982. It too would have gone another 2-3000 easily, and probably would have gotten me home, since I rode exactly 2000 miles from Exeter, RI, to Fort Morgan, CO. But once again, discretion seemed the better part of valor.
The rear I replaced in Gainesville now has 9,000 miles on it and is starting to square off a little bit. I don't have any more long trips planned in the near future and will probably ride it for at least another 5,000 miles.
Time, as they say, will tell.