First focus on your Board exam.As it is necessary to secure minimum 60% marks in the board to be eligible for IITs. Once you are confident about your board exams then start taking the test you have to balance between these two. In my opinion give at least 2-3 hours for board on daily basis and rest of the time you can devote to IIT JEE preparation. Remember IITs are in the process of taking admission on the basis of board. So don't ignore it.
All the best
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