Well, you don't have a standard answer to this question. Providing a timeline for the same is actually difficult. It would vary from student to student.
I am assuming that you have very strong reasons/aspirations for pursuing an MBA and thereby taking the CAT exam.
Yes, before you start preparing for CAT, do your homework in the first place. Read through the syllabus for CAT in detail. Try attempting any previous year's CAT paper. The idea is to familiarize yourself with the topics tested in CAT. By familiarizing yourself with the syllabus and paper pattern, you will be in a position to determine the effort required by you to take CAT. This will help you in determining the time required for preparation.
Of the three sections, preparation for the Verbal Ability section requires time. Remember, 70% of the weightage for Verbal is through Reading Comprehension. If you are an avid reader, it works well else you will have to develop your interest in reading which will take some time. Improve your vocabulary and keep practicing.
The section on logical reasoning and Data Interpretation requires extensive practice. Familiarize yourself with different types of data sets. Choosing the right data set is very important. The wrong choice will result in getting four to five questions incorrect which can be a game-changer. Ideally, you should target four to five data sets. As mentioned earlier practice is very important. Lot many times you can get it right by merely analyzing the answer options.
Quantitative Ability has been the noire for most of the students. Please remember, the concepts tested in this section pertain to X-grade mathematics, which everyone has studied. True, CAT requires you to be able to apply these concepts. This can be achieved by regular practice.
Now, keeping in mind the above points, for an average student, eight to nine months should be good enough time to prepare for CAT.
The first four to five months should be devoted to clearing concepts and familiarizing with the various concepts tested in CAT. Around April - May start taking mocks. Basis your performance and thorough analysis, work on specific topics. Identify your strengths and weaknesses. Build on your strengths. Try to eliminate your weaker areas. Two months before the CAT, you should have identified a strategy that works for you. One month before the CAT exams consolidate on your strategy.
Ideally, you should be joining a coaching class. This helps in being systematic with the preparations. You will have access to relevant study material. Most important, you can seek guidance from the Faculty. You can consider joining Career Launcher for Classroom training. The faculty at Career Launcher is really helpful. The study material provided is very much relevant and takes care of almost all management entrance tests. The test series offered by Career Launcher is simply amazing. The difficulty level of the mock test is at par with the actual exams. The analytics provided are very helpful.
Remember, flexibility in your approach towards CAT is desirable. Perseverance alone will take you through your rounds of preparation.