I think you’re being generous, many young people do not have any support at all for their education, financial or otherwise.
BUT …
(there’s always a but)
If you have the means to provide more financial assistance, I would point out that 20 hours of work is a lot of time for a student. In addition to time needed for studying, 20 hours of work per week doesn’t allow for a lot of time for college activities outside of the classroom. This is a big part of college, and it’s unfortunate that many students cannot take advantage of the programs offered at a university because they must work. It is difficult to fully participate in student government, for example, if she’s expected on her shift at the local Denny’s. Current institutional research tends to the notion that the students who are the most engaged on campus are often among the most successful in their academics.
There are other 20 hours worth of things to do that might have more value in the long-term. An exciting, unpaid internship in the field of her future career might be more important over the long term than 20 hours of paid work doing the filing in the admissions office. 10 hours of paid work plus 5 hours of volunteering in a literacy program might also be a learning experience – college students are often in demand at service agencies because of their flexible schedules, and once she enters the workforce, it becomes much harder to find 5 hours per week to devote to a volunteer activity.
The type of work can be important, too. If she is offered a lower paying job as a research assistant to a professor in her department, I would argue that it’s better for her to do that and receive some additional “income” from you than to take a higher paying job as a cashier in the dining hall.
My humble opinion is that you present the deal, and then offer her the opportunity to present her case at some point in the future should she find that she has 5 or 10 or 20 hours to fill with a worthwhile, educational, unpaid (or low-paying) activity.
Good grief, this is so preachy. From the OP, I get the sense that the issue is not a lack of finances from you, the parent, although I could be completely wrong about that. Obviously, some students’ situations dictate that they find paying work, and these people can, and do, do very well in their academic careers.
This is all IMHO of course, and based on 13 years of working at a college.