How long will it keep? Am guessing I may not have any leftover to keep, but...

Another standard technique for such syrups that is used by craft soda makers is to add a small amount of alcohol to help with the keeping qualities. Add about 1-2Tbs of alcohol (in this case use whiskey since you use it later--otherwise use a high proof neutral vodka) to the syrup once it is cooled and stir to distribute. This amount is well below the accepted limit of 0.5% ABV for a non-alcoholic drink.

Haha! I’m with you, I usually run out before I’d consider it risky. But to answer your question, because of the high sugar and acid content, boiled cider should keep for at least 3 weeks in your fridge, and pretty much indefinitely if you sterilize the jar or bottle first (just boil it as you would for canning).