At your age, probably just a couple hours a week would be enough. Working is pretty meh, and there are a lot of other things that are far more interesting to do. Money is really nice, but you'll probably appreciate your non-working hours even more. : D
I suggest putting away a percentage of whatever you make for long-term goals, like being an exchange student. If you get used to not spending the portion of money, you won't even miss it.