We charge our children 10% of their monthly gross income, unless they are in school (college). This is a better deal than they could get anywhere else. But we want them to be accustomed to paying rent, like other adults.
I don't know where you live, but to arrive at a fair rent, look at craigslist in your area for rooms to rent in houses. Your parents might give you a discount off of that. How much you stay in the room or what resources you use are irrelevant. A stranger renting the room would pay the same whether they slept there every night or not, and a stranger would probably not be eating the family's food, either.