Different experts / students will give different answers to this question. You cannot simply rely on the opinion given by an Individual, unless that individual has clearly analyzed your strengths and weaknesses.

For example, if you are an Engineer or a student with strong Mathematics, you may require about 5-6 months to crack CAT, assuming that you have decent reading habit. However, the same engineer or Mathematics background student may require 10-12 months to score 98+ percentile, if he/she is not very good at Language.
Similarly, a Non-engineer with an average understand to the mathematics concept may require 10-12 months time to prepare as almost 50% questions in CAT exam requires you to use Mathematical Concepts – directly or indirectly.

So, if you are one of those who need 10-12 months time, you can start your preparations from Dec or Jan month. (For example, if you are planning to take up CAT in 2020, you can start your prep from Dec 2019 or Jan 2020). And if you require about 5-6 months of preparations, you can start your preparations from May or June month.

In any case, Make sure that you complete the syllabus latest by Sept month. Then, you can just focus on writing mock tests of CAT and other Entrance exams. Do not overlook other exams as you will be left with very little time after CAT for exams Like SNAP, NMAT, etc.

It is also advisable to take mock tests in the period June-Sept, when you are actually developing understanding to the concepts. No matter what score you obtain in Mocks during this period, it will definitely give you idea about the paper patterns of various MBA Exams and also signal you about which concepts or variety of questions should you focus more.

For any assistance related to Preparations of CAT or other MBA Entrance exams, you can contact Proton Training solutions, Pune.