The reason wine doesn’t last after it has been opened is because it comes into contact with oxygen which leads to a chemical reaction known as ‘oxidation’. This process may initially soften and open up the aromas of the wine, but as it continues it will make it undrinkable.
One indication that a wine has become oxidized is that its colour changes. White wines become darker, rose wines turn orange and red wines go orange or even brown. The nose on an oxidized wine will also give it away, the aroma will be less potent, sometimes almost becoming undetectable. The final giveaway will be the taste. Oxidized wines seem dead, with little or no flavour, and actually be unpleasant to drink.
Different wines oxidize at different speeds and some need to be exposed to the air before drinking. There are wines that need to breathe for 12 hours or so to reach their full tasting potential. Other wines are oxidised intentionally when they are made and this change in properties is incorporated into the style of wine. For instance, white wines from the Jura region in Eastern France are made from the Savagnin grape which is intentionally oxidized, this means they can be potentially can be left open for weeks without losing any of their original qualities.
As a general rule all wine should be drunk within a day of opening. Although some may last longer, it is a good idea to always taste the wine before drinking. If the wine tastes fine and as it did when first opened, it will be drinkable.
Keeping wine is made tricky because it will not change immediately from good to bad. The wine will begin to progressively develop tastes that are unpleasant which different individuals will detect at different stages.
There are methods that can preserve wines once they’ve been opened, such as vacuum pumps (Vacu vin is a popular brand) to remove the oxidising oxygen and inert gases (Private Preserve is the original wine preserving gas) that form a layer over the wine that the oxygen can’t reach. There is some debate about the value of these methods however, some say that pumping out the oxygen from a bottle of wine also extracts important flavours.