I've been at both ends of the scale having had an elderly parent and an adult child living with us (not together thankfully. That would have led to world war 3).
Both paid their way. When gran was with us she would pretend to forget, and I would find money in strange places, in little pockets in my handbag or even in a shoe of mine. There was no point in arguing with her (or even trying to give it back) so I would make little treats for, do her hair, nails, take her out to lunch etc. She often told me no money could buy the care she got from both me and OH.
We suggested to our son when he came home from abroad that as we did not know what would be an appropriate rent, that he should check the rents for one bed apartment or house share in the local area. He was delighted to accept that rate but had all meals plus phone electricity etc. No washing or ironing.
He since moved on to his own place.