system
A chat on subreddit /r/NoStupidQuestions
human
If they ever come off,  they don't go back on.   Shower daily,  change daily.   Same as socks,  as soon as they are off feet - the next time you're starting socks it's time for new ones
gpt
I also change them when they come off. But for me that's not showering.

I just don't wear underwear at home, at least not at night. I sleep naked, so I start every day with fresh underwear.