It's important to know when ... and why ... to take study breaks. 

Here's an example: me.

Nearly every day I spend several hours in front of my computer, grading law school exams and writing pages of advice to the students who wrote those exams.  Although the time varies considerably, I suppose I average between 45 to 60 minutes for each essay exam answer I work on.  

Coincidentally, that seems to be my optimal time-on-task before taking a (very short) break.  If the weather is right, I'll step out on my balcony and breathe in some ocean air, stretch a bit, and get back to my desk.  Other times, if I notice I'm becoming a bit lethargic, I'll spend a couple of minutes stretching ... if it's still morning, I may pour myself a cup of coffee, then head back to my desk.

Also, note that the "mental break" is just as important as the physical aspect.  Get your mind off the subject matter.  Relax your brain.

Sure, it's different for each of us.  And there are times when breaks are nearly impossible to sandwich in. For example, I don't know what the rules are where you'll be taking your bar examination, but when I took the exam in Glendale, California in 1974 (yikes!) examinees were allowed to stand up behind their chairs. Because in those days the exam consisted of three days of one-hour essay questions (that's right, 24 essay questions ... no MBE, MPT, and no MEE), standing, stretching, and deep breathing were as important as fast typing skills.  Those who had not practiced their timing (to make sure they had at least a couple of minutes between essays for stretching and deep breathing) were often pretty burned-out at the end of a day of testing.

Pay attention to what your body, mind, and spirit tell you when you study.  Always shoot for optimal performance by taking those short breaks when you need them.

(Secret: I just wrote this while taking a break!  Now it's back to work!)