Hello Sahil. Well to write the CAT exam, one should be in the final year of graduation or should have a graduation degree. Now, the coaching duration differs from person to person. Few can score 99+ percentile with preparation of 1-2 months, while some can't even score 70 after a year long preparation.

I would suggest you to write a free full length mock test of a coaching institute. See your results, and if your performance is poor, then start coaching from october 2019 for cat 2020, or else , if you score well, then start preparing from June next year.

Hope this helps.

All the best.