I've found studying for 30 mins and then taking 40 or so mins as a break and repeating that a few times works for me. I think a lot of people just cram everything and forget how important it is to take breaks. The longer you study the less you will retain, at least from what I see.
In terms of study methods, I do a few different things. I first read the unit I need to study, then I write out all the key terms, lastly, I read these things aloud, almost like teaching an imaginary class. When I do this part I try to put things in my own words the best I can.
I've found this to be the best routine for me. So far I've been maintaining the best grades I've had by implementing different study methods plus breaks.