I study 4-5 hours every day (including weekends) and then like 6 the week leading up to an exam. I like this though because it means I don't have to stress for exams since I've kept a strong pace throughout. I personally prefer to work moderately over a long time than totally be miserable and stressed with 8-10 hour study days before an exam.
It's common sense: the more you study, the better you will probably do. Some people are literally studying 14hrs+ a day while others are studying 1-2hrs per day. Do what you need to do to reach your goals.
I averaged 8-12 hours on a regular day during first and second year, but i gave myself one day off per week. During Step studying i ate, slept, and studied for weeks on end.
Diminished returns was more a feeling than an actual result. I knew that if it was taking me 3 hours to outline 1 lecture then i was wasting my time and i stepped away and watched youtube to recenter myself.
Just FYI, there will always be someone who studies less than you, finishes their exam before you, and gets better scores than you.
We just started basic science today, so we are studying like crazy (est. 6 hours today not counting lecture) now since almost everyone wants to get off to a good start. Back to muscles, arteries, and nerves of the back and shoulder...
MS1 here who just passed his first test. I don't go to lecture anymore (waste of time for me). On an average weekday I spend 3-4 hours reviewing new lecture material taught that day + 1-2 hours using outside resources to clarify/cement concepts, weekends I'll do 4-5 hours each day reviewing everything from that week and do 1-2 hours of online flash cards, week before a test I do about 8 hours a day reviewing + 4-6 hours reviewing outside material/online flash cards. When I hit burn out (which I know I have if I can no longer recall what I read 2-3 slides prior), I stop for an hour at least before picking up my work again. I find that allocating time to rest, relax, and work out helps me stay less stressed and retain information significantly more than trying to pull 8-10 hours a day studying after lecture (did that beginning of first first week, hated my life and didn't remember jack because I was studying in burn out against my brain's wishes). In general because of how lectures are spaced at my school, I always have 1 day a week completely off from studying, most of the time 2 days, and one day is usually lighter because maybe only 1 lecture that day was hard learning.
It's pretty variable, probably about 2-6 hours per day with a day or two off each week. We get a week off for the final exam each block and I run 14-16 hour days for about 5-6 days leading up to the exam. Yes, those weeks are the worst ever.