Place 1 (48-ounce) bottle of prune juice, preferably with pulp, in a 5-quart nonstick pot over high heat and bring to a boil. Lower the heat and cook until the liquid is reduced to 1½ cups, about 50 to 55 minutes. Let cool, then place in a covered container and refrigerate.
Before using the syrup in a recipe, remove it from the refrigerator as you start your preparation, so it will have time to come to room temperature. The syrup will be thinner and easier to pour and measure.