Every family's different. I (as a parent) probably wouldn't enter into something that loose, asking my kid to help out with what's on the shopping list, eating out, and their own expenses, but it would depend on what everybody expected to get out of this arrangement.

Casual agreements work best when both parties act like responsible adults.

My SS lived with us for a few years after college. He was responsible for his expenses. If he didn't like what food was available, he could buy what he wanted. We thought it was better for him to save for a downpayment than pay us rent. There was no drama over the arrangement.