How long should cycling shorts last? As a follow-up to "What should I look for in a good pair of cycling shorts?," how long should I expect a good pair of cycling shorts (regular or bib) last? I think that an estimate in hours or km would be more useful than months or years(?). Is there an objective way to know when I should replace a worn out pair?
Eventually, the material will thin out and holes will start to appear. Objectively, if people can see your junk, then it's time to retire the shorts (or, in colder climates, restrict them to winter-only duty under tights). The chamois will deteriorate over time as well, so if a pair of shorts isn't as comfortable as it once was, it's about time to throw it away.

As far as durability, I'd be very disappointed in any set of shorts that didn't last at least 5000 km. 

Another note on care and feeding--make sure that the shorts never rub against anything abrasive (seat bag, velcro bits hanging off jacket, etc.) while you ride, as the material can wear through in an awful hurry.