Cats who bite during play: where do you draw the line?

Hands are not toys, and yet a lot of kittens are taught otherwise in their first weeks at home, usually by well-meaning people who found it funny at the time. Undoing it takes consistency more than cleverness. We stop the game the moment teeth touch skin, wait, then restart with a wand toy so the cat still gets the hunt it was asking for. Where is your line? Do you allow gentle mouthing, or nothing at all? And if you have retrained an adult cat out of biting, how long did it take? Background reading: why cats bite and indoor enrichment