There is no limit to study hours. If you just want to crack IIT you may not need to study for many hours a day but if you are looking to score a good rank in IIT then you will need to study a good number of hours. The best I can say is you should study the maximum comfortable hours you can give to study with your full focus and interest. The reason why I am focusing on concentration and interest is because if you try to think in your head, what memories you have of boring instances and what memories you have of interesting instances, you will find most of it is interesting instances which you wanted to make a memory of. Only those things reside in your head which you want to store( consciously). If you just give study hours for the sake of study hours, solving 100 questions in 4 hours then not even remembering next day what exactly you did yesterday. The best way to see how much focused you were in your study hours, you can revise(overview) the things studied during the day in night before sleep all within your head. And once in end of every week( complete week overview).
If you go up the rank chart in IIT JEE results in form of group of ranks you will mainly find the difference in study hours. There are other factors as well, retention, IQ, problem solving skills. Everybody has basic IQ in their subject of interest. Retention and problem solving skills depends upon how well you utilise your study hours.
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