There's some pretty compelling evidence that drinking coconut water in the morning is good for you and your body, especially if your goal is to stay hydrated. So I wouldn't blame you if you went out and bought dozens of boxes and bottles of coconut water in an attempt to get your hydration game right. But unlike regular bottled water, which has a theoretically indefinite shelf life, coconut water can go bad—especially if you don't store coconut water properly. So how long does coconut water last, and when does it expire?
If you're buying coconut water in those tall, cardboard boxes, also known as Tetra Paks, it should be be shelf-stable. That means you can store the unopened box of coconut water at room temperature for about a year. Each Tetra Pak of Vita Coco, for example, is labeled with a listed best-by date of a year after the production date. But one you twist open the top and expose the coconut water inside that box to air, you have to store it in the refrigerator, and you should probably drink it pretty quickly. "For best taste, after you open a Vita Coco," recommends the brand in their FAQ section, "place it in a refrigerator and consume within 24 to 48 hours—nobody likes sour coconuts."
You'll know if your coconut water has gone bad, though. The folks at Harmless Harvest, a company that bottles unpasteurized coconut water, note that their coconut water "can sour or have other changes in flavor, aroma and appearance" when left out at ambient temperature. That's, in part, because their coconut water "contains absolutely no preservatives or additives," and so "it must be maintained in refrigerated conditions at all times," not just after the bottle has been opened. But those changes—including a rancid odor—are a sure sign that you probably shouldn't drink that coconut water, no matter where it's from.