How Long Boxed Wine Lasts Once It's Opened
Boxed wine may not be fancy, but it serves a very important purpose: it's cheap. In fact, there's no actual difference between wine sold in boxes and wine sold in bottles — you're paying less money for a more portable container that includes a built-in tap. Sommeliers may scoff, but if you need wine and you're on a budget, then you get the last laugh, and you'll be laughing giddily once you quaff all that boxed vino.
There is one thing to always remember about boxed wine: unlike bottled wine, which can last a couple of years or longer so long as it's unopened, boxed wine has a limited shelf life from the moment it's packaged, even before you first start pouring it. The "best by" date on a box of wine is usually about a year away (assuming you bought it fresh). That box will then last for about three to six weeks after you first "tap" it — before enough oxygen slips into the semi-permeable container and slowly transforms the wine into vinegar. There is no such thing as vintage boxed wine (thus the scoffing wineheads).
Wine on tap
While a sealed wine bottle lasts longer than a sealed box, an open box of wine lasts much longer than opened bottles. This is because a glass bottle offers stronger protection than cardboard and the plastic bag inside the box, but the latter can be much more easily resealed to be airtight. Boxed wine containers have a pressurized vacuum inside them, which, particularly for a crowd, beats a bunch of fussy wine corks.
Once you first open it, the wine will start to oxidize faster: this means oxygen is messing with the ethanol in the wine, and as a result, the wine will turn darker in color and its taste will go flat. The wine's original taste will start to fade in about three weeks or so, but if you don't mind it going slightly stale, it should still taste alright for about six weeks in the fridge before it goes completely off. Still, that's a longer time frame than an open bottle of wine. Once opened, a wine bottle only lasts three to five days so you'll need to drink it quickly.
Can (and should) you freeze wine?
You can drink from a boxed wine container over a decent period, but if six weeks isn't enough and you don't want anything to go to waste, you can always freeze leftover wine in ice cube trays (for about three to six months), and plan to cook with it. Leftover boxed wine is a great choice for cooking or baking, as it can help form sauces or go into desserts as easily as any bottled vintage.
You might also consider keeping and freezing some boxed wine for cooking purposes because you get a lot more fermented grapes for your buck: a standard wine bottle contains five glasses' worth on average, but a three-liter box of wine contains about twenty glasses' worth. Considering most popular boxed wines go for about twenty to thirty dollars or less, it's not a bad idea to pick that over a pricier bottle if you just need the flavor for a recipe.