it depends on a whole lot of things, like how long it takes to travel to/from uni each day and how much uni work you do at home, and how much you stress about stuff.
i don't do much work at home for uni and am pretty relaxed about it (except right before exams), but i have a long train/bus commute; and i found i struggled with 15 hours per week.
for me, 8 hours on a saturday was ideal because it wasn't too stressful, and gave me a bit of spending money for the week
i reckon doing a bit of mundane supermarket work is good, because it reminds you why you are going to uni
hope this helps
