Discard milk if it is left out at room temperature for more than two hours. Milk that remains at room temperature for an extended period of time grows bacteria and should not be consumed.
Place milk at the back of the refrigerator rather than in the door or the front of the refrigerator due to fluctuating temperatures. When stored properly, opened and unopened milk lasts around a week past the "sell by" date. While proper storage helps extend the shelf life of milk, it can still go bad. Typically, spoiled milk has a sour smell, features a lumpy texture and may look discolored.