As per mayo Room temperature. Freshly expressed breast milk can be kept at room temperature for up to six hours. However, use or proper storage within four hours is optimal. If the room is especially warm, the limit is also four hours. 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, use or freezer storage within three days is optimal. Deep freezer. Freshly expressed breast milk can be stored in the back of a deep freezer for up to 12 months. However, using the frozen milk within six months is optimal.