What should be the best timetable for a student preparing for bits?

It depends on various factors like the student’s potential, time left for the admission exam to BITS-BITSAT, simultaneous preparation for boards, etc.
If you belong to 12th, 5 hours of rigorous daily preparation for BITSAT will ensure a very good score for you. Special attention should be given to preparation for English and Logical Reasoning as well.
If you are a drop-out, more stress must be put upon the number of hours of study, mode of preparation among other factors as much is at stake for you.
You should start solving mock tests available online within a couple of months at most.
Last, but not the least, an hour or two of recreation is most beneficial for aesthetic purposes. All the Best.