It depends on how fast learner you are. For some students, it takes 4 to 6 months for preparations for some others it will take years. However, if you are regularly studying NCERT books you are on the way of preparing for BITSAT. But NCERT books alone won't help you with the preparation you also need to solve problems from other books and learn the concepts.

I hope my answer helps. All the very best for your future endeavors!