Not just the amount but the quality of your study session matters too. You shouldn't study for more than 8 hours a day, because above that point your brain quite literally cannot take in any info. Answer these questions before you start studying:
- How long will this study session be? (Between 30min - 2h is the best) Can I do it uninterrupted? Clear all distraction from your sight.
- What part am I studying? What larger section this part belong to? What connect it to the other part? (For example, Stark effect belong to Electric phenomenon, and it is analogous to Zeeman effect which is a Magnetic phenomenon. )
- Why am I studying this? Do I completely have 0 idea, am weak at this part, or just perfecting the skills? At the end of the study, what should I achieve?
- How much should I study and by which method? What do I prioritise now? Practice what I'm good at or learn what I'm bad at?
- Separate what you need to learn into active and passive categories. Stuff in the passive category is usually harder to motivate yourself to learn, so make it active. And always understand where you are in the study road map.
- Set a bare minimum goal that you must absolutely reach even if you are tired af, a realistic goal that you can achieve studying normally, and a "Today I'm a hero!" Goal that you can reach if you tried your best.
- Stop every time you reach a goal, and ask yourself "Am I doing great? Should I try more, or it is time to move on to a different thing?" How is your study session quality so far? If it's good, why? Reinforce that feeling (mum said she's proud of me which boost my motivation, so let's repeat that scene 100 times in my brain!). If it's bad, why? What can you do to improve it? Is it so bad you want to stop today session? Would study something more lighthearted and fun help?
- Don't beat yourself up if you decide to stop, but don't let yourself off the hook all the time. Understand the consequences: my goals for today is to understand A, and because I skip, I won't understand A. You won't fail your exam, or lose to your friend, just because you skip today study. But you won't understand A, and if it keeps up, you won't understand B and C too. A might not be fun, or even important, but it is a part of your study and a bridge to your future, so let's give A the time it deserves tomorrow.
Don't revise blindly. You must understand what, why, and how you're doing. It's long to type out but it's only take minutes if not mere seconds to answer all these questions. Do it every hour you're studying, because your priority changes as you learn (eg this is too easy/ too hard compared to plans).
Some days I can study 10 hours a day and feel perfectly happy and healthy while doing it (of course, fatigue will hit immediately when I'm done). Other days, 2h is a stretch, but that's ok. It's all about setting the correct expectation, laid out all the steps, determine to finish your goals, and be proud of the results even if it's small.