Try starting out with 8 hours, preferably when your home to enjoy them (incase you work), and if you get a lot of algae, then reduce the time, if not and you want to view them longer, then increase the time. With a fowlr, light is only an appearance thing, not neccessarily beneficial to your fish. But as lion-craze already stated, whenever you do make up your mind, try to stick to a routine.
EDIT... I see your a student, so incase you work, I'm changing to... when your not in school.