Hi

First of all it's good that if  you are trying to study for a longer time but do make maintain the quality. By quality of study I mean that whatever time you spend studying, you should be focused towards your studies ,otherwise even sitting for 15 hours a day would be of no help as you won not able to get  what you are studying.

Follow the given below tips :-

* choose the time when your focus on concentration power is more.

For example,

There are people who prefer to study late night very well there are some people who prefer to study early morning

* do make a routine and make sure that you religious follow it ,routine will keep  a check on you and will help you to study for longer time

* make sure that you keep short break  in between your studies otherwise it would be brain tiring and you won't be able to focus

* practice questions only studying for reading a chapter can be boring so after completing each topic you should do the follow up questions to keep yourself active

*Keep changing the subject, otherwise it would be monotonous

* reward yourself :- after achieving your target of the day you can reward yourself and it can be anything like watching any episode of your favourite drama series or playing 20 to 30 minutes of  your favourite games or anything.

Thank you