From Dave Barry's Complete Guide to Guys:
When is it okay to throw away a set of veteran underwear?
a. When it has turned the color of a dead whale and developed new holes so large that you're not sure which ones were originally intended for your legs.
b. When it is down to eight loosely connected underwear molecules and has to be handled with tweezers.
c. It is never okay to throw away veteran underwear. A real guy checks the garbage regularly in case somebody -- and we are not naming names, but this would be his wife -- is quietly trying to discard his underwear, which she is frankly jealous of, because the guy seems to have a more intimate relationship with it than with her.