No idea, DH does them, I think once a week.

When I have been in charge I.e. Before lived together then mine got done whenever thought got a bit whiffy.

I hate it when hotels wash them after each use such a waste of water and energy although alot of them now have a system where leave in certain place if happy to use again.

When I was a kid my aunty used to visit and she would wash her families towels after each use - it drove my mum bonkers.