When a bottle is opened, oxigen gets to it and will start altering the wine. What the immediate effect is depends on the wine. Some, especially young wines will just speed-age and can gain from the exposure, while older wines will soon reach the point where they are damaged. When you first taste the wine and it is very young and effervecent, it most likely will not get damaged by oxigen quickly. If the wine tastes mature already, the oxigen most likely will have a damaging effect. And finally it's up to you how you like your wines to taste, young and fresh or rather more mature. Just be careful not to pass the point of no return, where the wine is overoxidized and bad.