How does a Wine Oxidise?
Typically red wines benefit from breathing, along with some whites, and generally the wines will improve with as little as 15-20 minutes of air time. By allowing the wine to mix with air, the wine will typically warm up and the wine's aromas will open up, the flavour profile will soften and mellow out a bit and the overall flavour characteristics should improve.
However, wine can begin to oxidise in as little as 2-8 hours upon opening the bottle. The biggest factor in oxidation is the amount of wine surface that is in contact with air following opening and pouring. When wine is left in an opened bottle, it rapidly deteriorates as the air in the bottle reacts with the critical components in wine that create its complex aroma and flavour profile. Eventually the wine may begin to look dull and will produce a lifeless wine that loses its unique fruit scents, and the smell may even remind you of vinegar.
The wine preservation systems work by reducing the amount of air that comes into contact with wine to prevent or slow down the oxidation process.