Wine evolves once the bottle is opened and has a finite lifespan. It is the gradual exposure to oxygen that ages a wine. Once you open the wine, you are accelerating this procedure. The higher the alcohol, the more robust the wine. Most half-full bottles of wines and champagnes will oxidize overnight. Use a vacu-vin to remove the air from the bottle.
Fortified wines have been stabilized with the addition of spirit, so there is a little more leeway.
Vintage Port: this is still maturing in the bottle, keep for 5 to 7 days;
Tawny Port/Madeira: this has finished its maturation in the barrel; keep for up to 1
Fino/Manzanilla Sherry: these fine, delicate Sherries should only be kept for 5-7 days.
Amontillado Sherry: can be kept for not more than a week.
Cream Sherry: can be kept for up to 1 month.
Bear in mind that the more exposure to air, the quicker the wine will become oxidized; i.e. the wine in a half empty bottle will oxidize sooner than a bottle with one glass out of it. Also, the way the wine has been made affects how quickly a wine becomes oxidized. Generally, the more naturally made, the sooner the wine will oxidize. Also, the older a wine or vintage port, the quicker they will oxidize, as they are towards the end of their life.