GMAT classes

How long does it take to actually prepare well for GMAT and get a really good score?

What exactly are you targeting by taking GMAT? If you are targeting a good school like ISB after your CA, make sure that you score at least 700+. GMAT score is just one part of your application, what is excessively looked into is your profile, acads, co-curricular, extra-curricular and other achievements. Global schools don't have a cut off like Indian schools, there are cases with someone getting an admit with just a score of 690+good profile and someone not making it with a score of 750+ terrible profile.

I want to make it to one of the best b-schools of India through my GMAT. What would be your suggestion to do?

ISB is the best in the country, then SPJIMR(Bhartiya Vidya Bhavan). 

There are several executive MBA programs from IIMs but you need a minimum work experience of more than 5 years.

Apart from these, if you are pursuing it from an aira gaira nattu khaira college by seeing the ad etc, you'll only lose money. Because of Covid, the placement scene is not that great.

Does SPJIMR accept GMAT scores?

How much time would it take to prepare for GMAT?