Many commercially available varieties of almond milk have been ultra pasteurized—heated very quickly 280°F and then cooled down just as fast—to extend shelf life. If it's a refrigerated carton, it's generally recommended that once opened, it should be used within seven days. Shelf stable almond milk, in addition to being ultra pasteurized, is packaged in sealed, sterile Terta Pak cartons to allow it to be stored at room temperature when it's unopened. Once opened, the carton should be stored in the refrigerator and most brands state that the milk should be used within 7-10 days of opening.