I am doing about 2 hours of revision every weekday and 4hours each day on the weekend. Many people will say to revise as much as you possibly can, but this isn't healthy revision. Regular breaks should be took every 30mins-1hour but this shouldn't be counted as your revision time. You should eat healthily, because it will raise the endorphins in your body which will increase your mood. This should also be done before the exam, especially if you're nervous, stressed, or worried.