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?
For shorts that I commute in every weekday I spend a little over an hour a day in them (100km/week). Those should last 3 months to a year when laundered weekly. Cheaper ones tend to last 3-6 months, better ones closer to a year. I've had some shocking experiences with branded cheap shorts lasting a month, and other times apparently identical shorts will last a year. I regard anything over 6 months as a win. But I buy commuting shorts out of the sale bins so my selection is limited.
I haven't trained on an upright for some years, but when I used to I'd spend a bit more and get better ones, focussing mostly on the chamois. A thicker, well shaped chamois normallys comes with shorts that will outlast the chamois so I buy on that. I still only get 6 months out of the shorts, but I ride 6-12 hours a week in them and at least rinse them after every ride.
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.
I've only worn out 3 or 4 pairs of regular shorts in the last 18 years, so it's possible to make them last. I try to buy high quality pairs and relegate the older ones for use on the trainer. Proper care (gentle cold water wash, hang dry) is important to keep them from wearing out too soon.
If the shorts develop large holes, lose their stretch or the stitching starts to come undone, then it's time to say goodbye. Also watch out for the chamois bunching up or getting rough in places.