I maintain a 2-week supply of boxers, but do laundry every week. Over 10 years theres roughly 520 laundry cycles. I change my boxers at least once a day. Assume each boxer has equal probability of being selected, so it creates a 50% chance of being in that wash cycle. So its feasible that they've been through ~260 wash cycles.
Right. And that's one wash every 36 days, so if he has a dozen pairs of underwear and "a few" means three, the math checks out for regular use.
Those underwears have so much character they can literally stand on their own.