I answered this in your other posting there is no specific rule for on campus hours unlike off campus where there is the 20 hour limit which is there for a reason which is that the primary purpose of issuing a study permit is to study not work. The ability to work is there to supplement any funding a student brings with them but not intended to fully fund any fees or living expenses.
In going over the 20 hours for example with on campus work then I guess it is a personal choice whether a student will lose focus on succeeding with their course but have a great work schedule so a balance might need to be set. This is a personal choice and everyones capabilities are different but not much use if come graduation a student fails due to taking on too great a schedule with study and work.
