how many hours a 12th std science student should study a single subject in one day.

Dear student, it can be better decided by you only that how long do you take to understand a concept and acordingly you may divide your study hours for that particular subject. There are students who are an early bird, i.e. they better understand in the early hours of the day and there is  another kind of students who are late night oil burners i.e. they better understand in late night hours when all the family members have slept and there is a pin drop silence all around. Thus, it’s purely up to you to decide when and for how long to study. I can certainly help you in selecting institutions, consider these names – SRM University, Chennai,.....................JJTU, Jhunjhunu.........................Lovely Professional University, Phaghwara