I don't think it matters how much time you do in the sense that you shouldn't have a textbook open for seven hours straight thinking that this is good revision!
Make sure you know what you want to revise (Eg. Techno Fix Topic for Geography) and revise that for 30 minutes, have a 5 minute break (make a coffee, watch a youtube vid, air guitar to Foo Fighters) and then repeat with a new topic!
I do 3 to 4 hours of this, works wonders :P