I worked 12 hours a week + occasional overtime during my AS year and managed to get 4 A's - so it can be done. Having said that, most, if not virtually all of my Sunday was spent doing homework and reading notes etc. If I didn't work, my weekend would be a bit more relaxed and less stressful but hey, I need/want the money.
I am, however, contemplating reducing my hours from 12 to 9 hours a week...
(Oh, a word of warning, I think you may find it hard reducing your hours or "stopping work" during exam time - it generally isn't that flexible.)