If you start preparing from now then it is very difficult for you to score 90 marks in KVPY SB stream. It is even more difficult if you have chosen PCM stream. Minimum 6 months of preparation is required for you to scoring the qualifying marks i.e., 50-60.

However, nothing is impossible. you have to make sure that your concepts are clear. Give equal importance to class 11th and 12th syllabus. Take special care for Maths if you have opted for PCM , Maths questions come really tough compared to physics and chemistry. If you have taken preparation for JEE Mains then it will be really helpful in KVPY too.

And practice previous year's questions and JEE level MCQ'S. Practice solving them as quickly as possible.

If you haven't done any of it ever then you may not get 90 but you can secure qualifying marks.

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