Boil cider in a 5- to 6-qt. pot over high heat, stirring occasionally, until reduced to 2 cups, about 40 minutes; "4 to 1 is a good place to start," says Davis, "but if someone wants a more intense flavor, they could take it further. You want the flavor to be concentrated but still bright." Let cool. Stir before using.
Make ahead: Up to 1 month, chilled, or 6 months, frozen.