Studying 12 hours straight is unrealistic.
Don't study continuously because it will only cause trouble for you that means you will loose your concentration and starts forgetting things... So the solution is study continuously for 30 min and then take a rest while of 5-10 min repeat this process ..this process will help you to study and concentration more...
All the best for your exams.
Don't try to study 12 hours straight - I GUARANTEE THAT IT WON'T WORK! Your mind and your body need to rest. I recomment you to try a techniqur called pomodoro, where you study for 25 minutes followed by a 5 minutes long break. After 4 pomodoro sessions, take a longer break.
Moreover, 12 hours is literally half of your day so remeber to find time for sleeping as well