4 hrs a day.
If you keep studying 4hrs/day everyday for a long period of time you'll be super efficient. People who do this 8hrs/day are robots, everybody can't do that. Normal people doing even for 6hrs/day quit super early. The brain saturates and unconsciously you want to skip it the next day. If you take it easy you're more prone to wake up and go in. With 6hrs/day it's too hard, the experience becomes too unpleasant and you become reluctant. Studies show that pupils can't even concentrate for 2 hours… educational system with hours of course early in the morning is bullshit.
I hope you're just not a lazy student asking this question because his exams come soon.
Because asking this question means you already quit in your head. Studying something for yourself because you want to learn is based.