When stored in the refrigerator, whole onions last approximately one to two months. You can refrigerate an onion that has already been sliced or chopped in a sealed container for seven to 10 days.
When onions begin to spoil, they develop dark brown or black spots. It is safe to cut out these spots, provided that you use the rest of the onion immediately.
Onions that are not going to be used before spoiling can be safely frozen. Peel and chop the onions, and store them in an airtight bag or container. You can freeze them for six to eight months.