It's important to take breaks to give your brain a rest from its hard work, it's recomended to take a break every 30 minutes of revision for at least 5 minutes, but it's important not to take to long a break as you wiont want to start again and may get distracted. Before re-testing yourself it is always good to wait over an hour to make sure it is really in your head and you wont forget it for your exam. Try making someone else, for example your mum or dad test you and ask them to do it in a different order or to re-word the questions to make sure you aren't caught off gaurd by the questions being asked in a different way than you have revised.