Cooled leftovers can be stored covered in the refrigerator for up to 4 days. Individual portions can be reheated in the microwave, or the entire casserole can be reheated, covered, in a 350-degree F oven for 15 minutes. The time can vary depending on how much casserole is left. Remove the casserole dish at least 30 minutes before reheating in an oven.