Studying hours does not have a hard and fast rules, few students study for 2 hours, few 4 hours. one thing a student has to remember whenever you are studying, you should be completely into it. if one is studying chemistry topic so concentrate completely on that [particular topic of chemistry, do not think about maths or physics during that time. revision after the study is very important, keep 1 to 2 hours for solving questions which you have studied. so 4 to 5 hours study starting form 11th class will be helpful for JEE.