Again, only you can answer that, which are your weakest subjects?
It's all about balance. Try and work on your weakest subjects but don't neglect/let standards slip in your strong subjects.
Sorry I can give you more advice. I didn't put any revision into my GCSE except for my German oral and one hour of history. I don't know what I spent the time on.
The only time I really made a great effort with revision wasn't until university. A small amount during my A-levels. Although at uni you're dealing with around six different modules it's a bit of a different balancing act than nine or ten GCSE subjects.