I like to take a break every hour, go exercise, go for a walk, do something physical. Don't watch TV, get on the computer, or start playing games. But! If you're not disciplined enough the "entertainment" will draw you in and hours will pass before you get back to studying.
Personally, taking my dog for a quick walk or hitting the gym with my books works for me. Another technique that seems to work well for me is to switch between subjects, ie, do an hour of studying for x, then do 30 minutes of reading for y, then another hour of studying for x, and a couple problems for y.
Anyways, everyone is different and what may work for me won't work for you. But, if you read a paragraph (or study a problem), then try to summarize what it says/does without looking over it again and you have no idea or you draw a blank, then you're not effectively studying ANYWAYS and you're just wasting your time.
In my opinion, it's not necessarily the TIME you put into studying, but the retention/learning of the material that comes with it. So, if you're one of those people who can only study for 20 minutes before needing a 10 minute break, then do so. Otherwise, as I said before, you're wasting your time. Experiment with your learning style and I'm sure you'll pick up alternative ways to study. :)