Get yourself the confuser!!! if you can come out on top in the papers in that book, you've pretty much sussed each subject.
Try and tackle your least favourite subjects exam first, usually Law, Met and Tech in that order. I learnt mostly by reading the text books for 40 mins before going to sleep each night, worked for me.
I don't think there's a fixed time u need to spend with your head buried in a book in order to pass an exam. It's all down to the individual concerned.
Best of luck.