GMAT

No matter what they say GMAT is important so stop asking questions like I have a 10 year experience can I do with 520 GMAT. Answer is no If you are aiming at a top school at INSEAD.Well you might have a chance if you are a NASA mission specialist or an Olympic athlete but otherwise no point even wasting your time on essays. GMAT tests your basic aptitude and if you can’t score on this test there can be two reasons either you are not smart enough or you did not take it seriously and both these reasons are not helpful.

So what is the right score well it depends where you come from and who are you competing against? If you are from India or China you better score above 700 because most applicants from these two countries have a 700 + score. Considering INSEAD’s diversity rule they will only select a certain % of applicants from one country and given that the number of applicants can be rather large from India and China competition gets tougher. So it’s better to start prep and take it seriously.
You should ideally put in about 4 months of Prep before you take your test. If you are aiming at October deadlines start in Jan so by May you are over with GMAT and you can put your time in essays. This also gives you time to retake the test just in case you didn’t score well. So my advice starts early.

Should you join a coaching centre depends on how self-disciplined are you if you are you can prep at home using various guides like Kaplan and other online tutorials but if you are like me who needs to be pushed joining a coaching centre is a good idea. They can teach you techniques which you might not be able to learn otherwise. In India I suggest go with Jamboree over Princeton, this is just from personal experience and can opinions vary. Also Kaplan guides are better for higher scores. Do as many tests possible and do them seriously and once you think you have prepped enough give the tests which are on the official GMAT site. They are the closest to the real GMAT and would give you your expected score accurately.