IIT JEE preparation can happen in 5-6 months time if you do it correctly. Most students try to plan things out at this stage but fail to execute them. There are many reasons why they fail. One of the reason is indiscipline. It is difficult to follow a path for such a long time. Another reason they fail is the plans are not realistic. All of a sudden they see a mountain of syllabus and make very aggressive plans of finishing topics in a jiffy.
Right way to make a plan and sticking to it is shown in the video below:
5-6 Months of preparation is possible if you do the right things. You do not really have to work 15-16 hours. You have to concentrate and relax. You also need to make your preparation more efficient so that you are able to make most use of the time.