Room temperature. Freshly expressed breast milk can be kept at room temperature for up to six hours. However, it's optimal to use or properly store the breast milk within four hours, especially if the room is warm. Insulated cooler. Freshly expressed breast milk can be stored in an insulated cooler with ice packs for up to one day. Refrigerator. Freshly expressed breast milk can be stored in the back of the refrigerator for up to five days in clean conditions. However, it's optimal to use or freeze the milk within three days. Deep freezer. Freshly expressed breast milk can be stored in the back of a deep freezer for up to 12 months, but using the frozen milk within six months is optimal.