This article gives great information about the topic of letting a good wine breathe.

What is wine breathing? It is exposing wine to air and aerating it. Oxygen is the element in the air that affects wine. White wines do not seem to react right away to oxygen, so when we talk about letting a wine breathe; we are talking about red wine. Practically all wine is crafted in a way that causes it to evolve over time.

If this were not the case, then we all would be buying and drinking month –old wine like we buy month-old beer. A large part of the evolution is the mellowing of the wine. Tannins and acids are the components that have the most need to mellow. Without sufficient exposure to oxygen, wines can taste harsh.

Eventually, as the tannins and/or acids fade a bit, the fruit begins to exert itself and the wine’s components become more balanced. After too much exposure, wine, especially red wine, begins to taste like vinegar. This is caused by the development of acetic acid.

A Zinfandel, Cabernet Sauvignon, or Shiraz may need to breathe for an hour or more, depending on how the wine was made and how mature it is. Equally important are the personal tastes of the people who will be drinking the wine. Some people like to taste wines right out of the bottle and experience evolution over time.

Generally speaking, all red wines taste better ten minutes after you open the bottle and pour it into the glass. Just taking the cork out and leaving the wine in the bottle is ineffective, since so little of the wine is exposed to air. Let your wine breathe in a glass. Wines that are still before their peak when the cork is removed may taste much better after half an hour or more in a glass. Letting your wine breathe does not have to be a controlled scientific experiment.

It is interesting merely to observe how bad some good wines may taste when you first open the bottle. Just keep in mind that a red wine will probably be better (and unlikely to be worse) after ten minutes of aeration. A white wine may also benefit from some air and is unlikely to taste worse. And there are some wines that actually taste better the next day! (It is important to note that these wines had to be in a recorked bottle and not left out for a day in a wine glass).

If you ever have an expensive bottle of wine fifteen or more years old, you do not want to give it much air time. These wines can change drastically literally from minute to minute. This is because a lot of the mellowing has already been done by sitting in the bottle for years; these wines may run out of gas an hour after the bottle is opened. However, that can be an incredible hour, in which a seemingly static bottle of liquid changes dramatically.

Don’t bother saving part of an old wine for the next day unless you want to experiment: It will probably be dead.