If a potato develops growths on the surface or changes color, it has gone bad and should be discarded. When a potato softens without being cooked, it should be thrown out.
Sweet, russet and white potatoes last for three to five weeks past their sell-by date if they are stored in the pantry. Sweet potatoes remain fresh in the refrigerator for two to three months. White or russet potatoes stay fresh in the refrigerator for three to four months. Potatoes that have been cooked or mashed can be stored in a refrigerator for five to seven days before they should be discarded.