I hesitate to chuck anything expensive, I just buy sdomething new and keep the old stuff "in case"

But it is really hard to tell how long a pair of shorts last, since I rotate 4 or 5 pairs, and of those 2 or 3 favourites are in heavy rotation. I agree that they tend to get sloppy and shifty once the fabric has stretched enough. They may be see-through as well, I guess, but I can't see it and haven't been told it, not to my face at least... I see it often enough on other people's butts though!

My winter AmFib tights are easier to gauge, since I only had one pair until they well and truly fell apart. They lasted about 4 winters of daily use.