A high-end telescope is a long-term investment that should last you a decade or more if you pick the right optics today. Avoid buying something cheap that you will have to replace in a year, as a quality setup brings the universe to your backyard.
Check the release calendars for major brands like Celestron or Sky-Watcher to see if a new model is launching. Often, the previous year's model drops in price during holiday sales, offering you the same glass for significantly less money.
That 15-month window is a deadline, not free money. If you have a balance left over when the 15 months are up, the card will start charging interest on the remaining amount at the standard rate.