Actually Port is not as susceptable to oxidation (the thing that ruins wine once it is opened) as table wines. The port is fortified with high proof brandy which acts as an anti-oxidant. Oxidation does not neccessarily ruin a fortified wine. In fact, I purposely oxidize all my sherries and some of my ports. One example of a very fine port which is "oxidized" is Galloway Pipe Port. This, and all good tawny's are amber(ish) in color due to the oxidation and additional tannins picked up via long term barrell aging. Madiera is another example of a "baked" wine.
A standard wine (10 - 15% alc.) should be consumed within 24 - 48 hours of opening. But ports and sherries can stand up to weeks of exposure to air.
FYI the wine bottle is the size it is (750 ml) because historically it is accepted that one bottle can be consumed by two people in a single setting so as to avoid the whole issue. This rule does not really apply to fortified wines however which can be 18-22% alcohol.