Those holding student visa had better not work over 28 hours a week. It is a little too complicated to explain how the immigration counts the hours ( it does not start from Monday), the longest working hour is 7 hours per day. As long as the visa holder obeys the rule above, working hour does not cause you any problem.
Even after the graduation of the school, if the applicant wishing to change status to working visa, over working will be the serious problem. In the long run, you should not over work.