I'm sure this is repeating a lot of the great responses that other users have given so far.
The time that you use for revision will only really have an impact if you use it in the right way. You need to identify how you best intake and retain information and based it around that.
Also, trying to keep things fresh - maybe not sticking to the same technique because you may become jaded to it and not every method of revision works for every subject.
Revision is a subjective thing, all advice can only really be general I feel, because it isn't really a one-size-fits-all situation. It may be trial and error, but find what is best for you!
I'm planning to be revising for 8+ hours starting from April holidays. If I take regular breaks between each session would that be ok? I'm planning to do 2 hours for each subject but every half an hour, I would take a 10 minute break in between the 2 hours. After 2 hours, I would have a half an hour break.