Hours are not important. The ability to recollect what you studied earlier matters.
Identify the part-of-day when you feel you can fully focus. Elders often recommend early mornings, but for that you need to be able to forego prime time TV shows. Only then you can earn a good and sound sleep, very essential for your body & mind to awake refreshed.
Divide subjects in (1) easy to grasp, and (2) difficult. Allot more time to the latter. Depending on how much you covered, re-allot time for other subjects in this group.