Preheat oven to 325 degrees. Stir together all ingredients in a 9-by-13-inch baking dish. Bake, stirring every 30 minutes, until dark golden brown and reduced to 4 cups, about 1 hour, 50 minutes. Let cool completely before using. Apple butter can be stored in refrigerator up to 2 weeks, or in freezer up to 3 months.