I did an hour-on/hour-off cycle when I was in school studying for exams each semester, and it was great. Working for an hour meant that I could nail a solid two

I did an hour-on/hour-off cycle when I was in school studying for exams each semester, and it was great. Working for an hour meant that I could nail a solid two concepts, and then go do something else for an hour, and then come back and spend an hour on some practice problems that used those concepts. I remember initially trying a shorter 30-30 cycle, but when a problem takes 20 minutes to do (by design) subbing out at 30 didn't really get much done. I also found it let me work much longer. I'd get to the library at 10am, work one-on/one-off until 11pm or so, go home and watch a bit of TV, and repeat - for three weeks straight, only interrupted by days when I had actual exams, on which I'd do about four hours off after the exam and get back to work.