(This is my first post on this board, I think, but this thread really struck me for some reason as needing a response.)
Honestly, I think the answer to your questions depends on the child. I have 17-year old twins. One of them does have a job as a skating carhop. She works anywhere from 26-35 hours a week, maintains a 4.0+ GPA while taking three college classes (Calculus 3, Physics 2, Comp 2), attends four tae kwon do classes a week and, yes, still manages to find time to crochet, paint, hang out with friends and family, etc. I have no problem whatsoever with her working as much as she wants, she thrives on being constantly busy.
OTOH, her twin sister is pretty much the complete opposite. She claims she is putting in applications, but she refuses to learn how to drive, so the places she can apply are limited. She is maintaining a 3.6ish GPA in two college classes (U.S. History 1 and Astronomy), but does not apply herself in a manner that suggests she could handle much outside work; I would be reluctant to see her work more than 10-15 hours a week.
Sorry for the novel, I just wanted to say that I think it really depends on your child's personality and work ethic.