How many years preparation is needed to score best in CAT. Any suggestions since I recently completed my class 12th with 90.2 %.

Ideally preparation of 6-12 months should be enough for cracking CAT. In case you are not too confident, you can start with basics of maths, reasoning and English. Once you get a strong footing over the sections, try taking full length mock tests and analyse where you stand. If you join coaching, your mentors will guide you on tackling weak areas. To know more on preparation, read the following article:

www.bschool.careers360.com/articles/cat-when-and-how-start-preparations