Store any leftover banana bread in an airtight glass container on your kitchen counter for up to 4 days. Keep the container out of direct sunlight so condensation does not build up inside the lid. If you want to keep it longer, slice the loaf completely and wrap each piece in plastic wrap. Place the wrapped slices into a heavy-duty freezer bag and freeze them for up to 3 months. Reheat individual slices in a toaster oven at 325°F for about 5 minutes until warmed through. You can also microwave a slice for 20 seconds if you are in a rush. Watch out for a sour smell or visible fuzzy spots as these are clear signs of spoilage.