It depends! Whenever your mind gets diverted or you feel your concentration level is falling you should take a break. Prioritising of work helps you concentrate well.
I usually take a break of 10-15 minutes.
I personally don't like taking breaks just because I know that I will get distracted. If I go to watch tv or on the computer for 15 minutes, that will turn into a couple of hours. So for me, I would rather just keep revising. However, I do set aside breaks for eating (obviously) and if there is a programme on tv I would like to watch. However, if I do take a break from my revision, then it tends to be ten minutes in my room where I have no distractions and can clear my brain.