15 Months is a lot of time. You could take the test many times over in that time

Actually, in my opinion, 6-8 weeks is enough, but the more time you give yourself, the better, If you can build up and keep the momentum going.
You can start from the very basics. Start off with mastering all the concepts in the OG and solving the questions. Take a mock test every few weeks to know where you stand, analyse the test to know in which sections you are the weakest. Once the OG is complete, then go on to other material. It's going to help you get familiar with the GMAT and its question-types. Then, instead of alternating between verbal and quant in a loop, focus on sections in which you are weak and focussing on removing the weaknesses. - How do you know where you are weak ? - By analyzing the tests you take every few weeks.
I studied by killing off the weak points and my prep time was 7 weeks. You are definitely going to do much better sir. I wish you the best of luck with the test prep.
Also, you have so much time, so you can prepare well for the next stages of the admission process. Build up your profile, attend seminars, research your favorite b-schools and see if they are a good fit for you. Also, you can work on securing fellowships or scholarships. Since you are a non-native, so you will be taking the TOEFL (which doesn't take too much prep-time), so you may want to plan when to take it and familiarize yourself with what is tested.
I hope as unorganized as my reply is, it is of some help to you. All the Best