probs around 30 mins then a break... i suggest rewarding yourself and then do another 30 mins. if you are going doing really well keep going till your brain can't take it, but drink LOADS of water!!! hope this helps!
you should revise for 20 minutes with a 5 minute break. the break doesn't necceserilly have to be somethinh like watching t.v or food. an ideal break is little things such as small hand games to keep your brain engaged etc. if you want more tips on breaks, revision times, revision tips and loads more, check out my "How To Revise Effectively" rescource :D trust me It's Worth It! :D