A good pair of noise-canceling headphones should last you five years or more. Focus on build quality and comfort so you are not tempted to replace them when the next minor upgrade hits the shelves.
Wait for autumn sales events when manufacturers often clear stock. Last year's model is usually identical in sound quality to the newest version, making it the smarter buy for your wallet.
That 15-month window is a deadline, not free money. If you have a balance left when those 15 months are up, the bank will start charging you interest on the remaining amount.