At least once a week, but I change individual towels in between if they are getting ponky (but still wash them once a week). Our bathroom is cold and damp so in winter towels take turns in front of the heater. Getting them dry quickly seems to keep the ponk-factor to a minimum.

Kitchen hand towels & tea towels get changed more often, every day or two usually, but I wash them all together when there's enough for a full load.

FWIW I think the once-a-week thing came from my Nanna and Grandma who both had once-a-week washing days. For Grandma this meant Monday was washing day and Tuesday was ironing day, Wednesday she visited, Thursday was shopping day, and on Fridays she baked in readiness for the weekend when people would come from near and far to eat her baked goods.