There's no set number of hours. The goal of studying is to understand the topics so it depends on how long that takes. It'll vary person to person, topic to topic. I know that that's not the most helpful answer.
That being said 8 hours a day seems like to much... I would be burned out in less than 2 weeks if I tried to do that. I'd only do 8 hours a day during the revision period.
Have you done any of these?