If you’re just adding milk to a vacuum-insulated mug, it can last about 6 hours without turning bad. If you add a few ice cubes to the milk, it can last even longer. If you’re adding milk to your coffee, once your drink drops below 140 degrees Fahrenheit, you have about 4 hours where it should be safe to consume. For this reason, some people choose to carry their coffee and milk in separate vacuum-insulated mugs with the intention of combining both beverages later.