Retention is achieved best when studying occurs as so:
Study for 30-40 mins, then take a 10 min break, and repeat.
The above has been proven as the most successful retention method, but is however not applicable to everyone, and not feasible to most.
I.E, when I study for my University exams, I'm not the type of person that does continuous study through the semester; instead a few days - week approaching the exam(s), I'll generally force myself to study for 5-10 hours a day. Granted, it's not the most advisable method if you're looking to retain most of the information long term, however it works out fine for me for retaining a decent amount of what I read and practice up till the exam.
In regards to study methods, actually practising questions and taking notes improves retention a significant amount. Simply only reading hardly compares to taking notes and answering practice questions relative to what you're studying.
Another thing I find personally useful is downloading my University lectures to my android and listening to them while running/bus/driving etc. I can't stand watching the lectures when I'm at home, so I instead download the lecture slides and audio, and read/listen to then on my android while doing the above mentioned.
