On the average, most manufacturers will say that a shorts life span in three years. Of course, at times that all goes out the window because we all are different in terms of how many days/week we ride, how many pair of shorts we own/rotate...etc. I use my shorts year round, so I've always thought that I'd save some of the 'older' ones for trainer use but only do that unless it's going to be a shorter ride. Unfortunately, there is no majic answer here, I've learned to buy shorts in pairs when I find a pair that I like and to do that quite often. For me, a good pair shorts is the insurance that I will go out and ride on the days that I simply don't want to do anything HA!