It probably varies by person. I take a 10-15 minute break every hour, with longer breaks towards the end of a long study day. Most of the time if something isn't sticking in my head I just move on to a related (or unrelated) topic and then things typically come together. I've never completely taken a day off to "process" information I just learned.. or really a whole day off at all. If we have an exam in the morning, for example, I will go home after the test, catch up on chores, then start studying the next block of info. I never truly feel burned out and I think it's because I study consistently everyday and therefore don't need to pull crazy hours during the week/days leading up to a test.
My classmates give me a hard time for "studying so much when the test isn't for another 2-3 weeks". Jokes on them though because I never feel stressed, burned out, sleep deprived, or like I need a break from school.
Also, I'm not the kind of person who can sit and memorize stuff and as such I don't make flashcards or anything like that (though I do use FC). I focus mainly on integrating topics and I find flashcards to be counterproductive in that regard. I take full advantage of spaced repetition and taking full days off would throw doesn't fit well into that scheme.
