13- to 15-hour days in a high-stress role is a real constraint, and I want to give you my perspective before anything else. You can prepare well on that schedule, but the calendar will be longer than it would be for someone with more room, and forcing a short timeline on top of that workload is how people burn out and end up retaking. Plan on a longer runway and build around what you can actually sustain.
Protect a small, fixed daily window and treat it as non-negotiable. Sixty to ninety minutes on a workday is enough if it happens nearly every day, and mornings tend to work better than late nights after a 15-hour shift. Leave the evening for lighter work: reviewing questions you already did, going back over a lesson from that morning, running flashcards on the commute. Weekends then carry the heavy load, since that is where new concepts get learned and full reviews get done.
Sequence the hours rather than scattering them. Start topic by topic: learn the concepts and techniques for one topic thoroughly, then practice only that topic until your accuracy is consistently high before moving on. Once a group of topics is solid, move into mixed sets, where you practice recognizing what you're looking at without being told the topic in advance. Both phases are necessary, and the order is what makes them work.
Then review every question you miss and name the cause: a concept you didn't know, a misread, a careless error, or a trap answer. Write down what you'll do differently next time. That step is where the hours convert into score. And keep in mind that focused study and total hours work together rather than substituting for each other. Neither one covers for the other.
Take one of the free official practice tests at mba.com early so you know the size of the gap you're working with. It won't give you a study plan, but it will tell you how far you have to go, and it lets you set a test date from your readiness rather than from the calendar.
If you want more on the scheduling side, this covers it in detail:
GMAT Study Plan for Working Professionals.
Plenty of people have earned strong scores on schedules like yours. The ones who get there do it with consistency over a long runway rather than heroics on any given night.
Genius001
As working at central bank, how to prepare for gmat? Working for 13-15 hours in stressful environment