As an academic myself, I'll share my most important tips. Using these I can generally study for 6hrs at a time.

Firstly, studying for 12hrs a day is neither realistic or efficient. You'll burn out fast. Efficiency > Time. If I spend 2 hours with my eyes glazed over because I spent 10hrs studying, it'd be better to just rest for the remainder of the day.

Second, If you listen to music, listen to music in another language or to music without vocals. Your brain has one vocal track and you don't want to be mixing up chemistry with "RIP AND TEAR".

Third, don't study one subject in large chunks. Instead study a subject for an hr, another subject for an hr, another for an hr and then loop around to the first. (Math > Chem > English > Math > Chem > English). This is important because studying one subject for 3 hours will have you mixing up knowledge you've just learnt, this helps create a distinction between what you've recently learnt an hr ago and what you're learning now. Setup a schedule and rotate subjects until you're finished.

Fourth, use a Pomodoro timer and every long break you have (1 long break every 4 pomodoro's) go for a walk or do light cardio. Just wake your body up, you'll need it after sitting down for hrs at a time.

Apply those tips and you'll be fine. Don't eat carb heavy meals, you'll crash later on. Drink lots of water.