So many YouTube Channels have tried to answer this most asked question by the students. A lot of YouTubers answer this question simply by subtracting the hours which would keep you occupied in regular activities by 24 and finding the appropriate number for you. They would argue, if you sleep 8 hours, eat for an hour, indulge in nature call activities for half an hour, go to coaching class which takes 5 hours etc, then you are left with 24-8-1/2-1-1/2-5 = 9 hours. Another 1-2 hours would get wasted here and there, So practically you are left with 6-8 hours to study. But this approach to answering this tough question has its limitations.
In the beginning if you study for IIT JEE for 6-8 hours , your study would not be very effective. Why? Because you would not be giving enough time to your mind to absorb the information you got from 5 hours of coaching and 6 -8 hours of self study. Its almost as if I ask you to run a treadmill for 4 hours. Effectiveness of this type of workout is questionable and obviously it would do more harm to your body than good. In contrast if I ask you to Study for half an hour, you would be able to do it with full concentration and focus.
In Studying, quality of study session matters a lot. Time you give to your body to absorb the information creates new neural pathways which help you retain the same information fast. Your mind absorbs information and creates such neural pathways when you sleep or when you indulge in light activities. This is also called the diffused way of thinking where you do not actually focus on the study material which you learnt, you purposely concentrate away from it and let the information settle in your brain.
Thus when you are starting your preparation you should have very intense short sessions of study filled with gaps in which you do leisurely activities. Ideally 25 mins intense study session away from distractions followed by 5-10 mins of recall and than break time are best ways to increase your study efficiency. When you are starting you can do 3-4 such sessions and than slowly and gradually increase these sessions as you build the stamina for more information accumulation.
Warning: You will get to know your threshhold when you try to study this way. Sometime less is more. Over studying can make your mind work slowly. Some great examples to illustrate this information are shown in this video