What school do you go to? My school separates PT classes and FT people, at least, I think they do anyway. They do for the first year.
Anyway, I plan on studying for four hours per day, part of that during work hours (I get many breaks), and part of it between work and school, and part of it after school. I hope four is enough, but I think six is recommended by my school for part timers.
I think the most important thing is that you decide how much you are going to study and then dedicate time to that every day. If you leave it all for the end of the day when you have time "left over", you'll find a way to do other stuff instead of studying.