well if you want to put a number to it...i'd say around 7-8 hours a wk...1 hour per week day, 2-3 hours on sat, and leave sunday as a break cause you deserve it lol...i say you need a lot cause by the sounds of it 40 ppl doing mx2 means it will get very competitive and the other kids in the class themselves will be putting in lots of hours a wk by going to tutoring or doing countless hours...so really you want to be doing some work each day (with one day break or you will burn out lol) so you can stay on top of the course and on top of your classmates.
When it comes to studying and revising it is best to have a nice summary of each topic with some general examples on how to do each type of question...then after that you do lots of basic questions using your summary if you forget how to do it...once you become familiar with the content you start doing the harder questions and eventually you practice the hardest questions you can find once you have mastered the content