Storage – Cover the leftovers well and keep it in the fridge for up to 3 days. If you don’t think you’ll eat the leftovers right away, they can be stored in the freezer for up to 3 months. To heat up the leftovers from frozen, allow them to defrost overnight, wrap a serving of the casserole in foil and bake it for 20 minutes at 350° to warm it up.