They typically do fail between 60K & 80K miles, but I certainly wouldn't consider them a maintenance item to replace at 80K miles regardless of whether or not it needs it. My original four O2 sensors have 91K miles on them and so far my mileage is as good or better than new with absolutely no driveablitly concerns. Using the logic of just replace them at 80K miles, I would have spent $250 plus the labor to swap out all four of mine 11K miles ago when it wasn't necessary. There are tests that can be performed on an O2 sensor to see how quickly it is responding and whether or not it is going above or below the threshold limits. Without the right equipment, it might make more sense to just go ahead and replace them especially if you only have two to deal with. Since I have the right equipment and four sensors, I will just keep checking mine every so often and replace them as they fail.
