When I was back in univ, I a minimum of 15 hours per week and took a full course load, 24 hours of lectures in my first year. It seemed to get easier for me as I went along though and by my final year I was doing 20 hours, all night shift plus a full course load and thesis. I still found I had more time in my final year than in my first year. My commute to univ was also a minimum of 1 hour each way.
Those hours at work were not wasted however because I managed to get a security guard job, which not only paid better than retail but allowed me to study. I also had plenty of time for a social life. I didn't have this luxury in my final year as the job I took was as a security person in a night club so I couldn't just whip out the books and study, however while working there my need for a social life was satiated as I had no will go to another bar or club b/c it felt like I was working.
My two hour commute each day was also well used, I read alot of the course material I would not have read otherwise.
So in summary I would suggest the following:
1) Get a night watchmen or security guard job, preferably were you are alone and noone will hassle you so that you can study AND get paid.
2)If you must work and CANNOT study while at work take on a job that keeps your social needs somewhat fulfilled, ie. a bouncer or bartender, even a waiter.
3) If you have a long commute to work, DON'T drive in yourself take public transit even if the ride is longer.