Unlike the complicated, nuanced process of winemaking, the technology behind boxing wine is quite simple. Wines are poured and sealed into either metallized or clear plastic bags, which are then sealed in a cardboard box. To open the box, you punch out a perforated disk of cardboard, pull out the plastic tap, and push a button or turn a knob to release the wine. Only a small amount of oxygen (the chief culprit when it comes to spoiling) can enter the container, which accounts for the wine's long shelf life.
But the shelf life of an unopened container is not boxed wine's strong point. Glass bottles are completely stable and nonreactive; metallized and clear plastic bags are not. Once wine is sealed in the bag and the box, it has only a year or so before it begins to deteriorate. Look for a packing date or a best-consumed-by date. Most worthwhile boxed wines will have one, or a code that can be deciphered by the company if you call it.