How long does an open bottle of red wine keep?

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Dear Dr. Vinny,
How long does an open bottle of red wine keep?
—Glen, Toronto
Dear Glen,
As soon as you open a bottle of wine, you expose the wine inside to more oxygen. While exposure to oxygen might initially help it become more expressive, eventually all wine sill start to fade and the flavors and aromas will flatten and start to taste oxidized—nutty and dull.
Bolder, younger wines will last longer after opened than delicate, older or light-bodied wines. Wines with higher acidity or residual sugar also have a longer life. It depends not just on the wine, but the person drinking it and their sensitivity to such things, but in general I think it will still taste good for three to five days after being opened, possibly longer, and depending a lot on how the wine is stored after it's been opened.
At the very least, reseal the bottle with the cork or screwcap, or find a stopper, to prevent further oxidation. You can also transfer the wine to a smaller bottle with less surface area. Storing an open bottle in the refrigerator (yes, even for reds) will also slow the oxidation process.
—Dr. Vinny