I paid 25% of what I earnt after I had finished full time education.
When my parents moved out into another house, I didn't give them rent (the house was owned outright) but I paid all of the bills for the upkeep of said house.
I don't think its unreasonable at all, if you are earning, to contribute to the household you live in.
I'm 47 though, so maybe its a generational thing, my step son (21) lives with us, does nothing about the house, works 5/6 days a month with his dad (bodyshop/refinisher type stuff) and earns enough to go out with his mates every night and get stoned, go on holiday a couple of times a year with his mates or girlfriend, has a gym membership, pays for his XBox membership thing, has all sorts of gym related shake stuff, eats loads (he's bulking apparently) and we struggle to get £50 a month out of him...
His girlfriend also lives with us, she was running her business out of our livingroom for about 8 months when she moved in as she was saving up to rent a salon or something, she 'doesnt see why she should pay board'...
I'm astounded by the shear entitlement of being able to live in someone else's house because your parents are going through a messy divorce, running a business from their livingroom (eyelash ****), wasting loads of food (shes a fussy eater, will say she likes something, will go and sit in their bedroom to eat it, but then not eat it and it'll go in the bin) and not be able to see why you should at least contribute...