Right now I'm doing three shifts a week, which is a lot (I will definitely cut down towards exams), however I do not do much else outside of school and I find in a way it helps me manage my time better. It's strange, I know, but it makes sense if you think of it like having less time forces you to manage your time more efficiently to get everything done. This really helps me because I'm the kind of person who will sit at their desk for a whole weekend - and will probably not get all my work done because I let myself get distracted with the mindset that I can dawdle because I have a lot of time.
Contrastingly, when I know I have work after school, I'll go to the library for 1-2 hours and get through all my work without any distractions. Personally, I find lots of short bursts of really solid work to be more useful than a whole weekend doing one thing really slowly.
It's all really just a personal thing everyone has to decide themselves, but this is just what I found to be helpful to me

xx
P.S. I've still been happy with the marks on all my SACs. I don't think I could have gotten better marks even if I had more time without a job.