It's about the amount of things that goes into our head rather
than the time 
Like for doing question, if you do a question and you can do that
question easily, mean that you are have clear that question or
topic...
If not, learn from that question and after learning, you learn
something new...
You will need to be able to answer any question taken from
anywhere... If you are able to do that, good grades is a easy
thing...
And btw, there is a Homework
Forum which includes studying...
Good luck 