This can vary tremendously as there are so many variables to take into account – but one thing is for certain - it is much shorter than many realise. So the following information is to ensure as best as possible you enjoy your Port at its best and don’t miss out on all the character in such an amazingly complex and dynamic wine.
A rough but simple guide: Any Port with a normal full length cork (one where you need a corkscrew to extract) should (when stored in a cool place) be consumed within a two-three days to enjoy it at its finest. Any Port with a T-Stopper type cork (can be removed by hand and easily replaced in the bottle) can generally be kept, securely re-corked in a cool place or in a cellar, for a few weeks without significant deterioration of the wine.

 

Below are more specific recommendations for several styles of Port.
Vintage Port:
Young Vintage Ports (less than 5 years old) can often last 4-5 days once opened. However, older VP’s (more than 15 years old) are not meant to be left open for more than 2-3 days. They won’t spoil if left open longer, but they will lose their freshness and seem a bit more subdued than they did when first opened, especially the aromatics. Really old VP’s (more than 25 years old) are at their best if consumed within 24-48 hours. Treat Vintage Port like a red wine.
LBV Port:
Unfiltered LBV Ports, if stored in a cool place (store approximately 6°C to 10°C) after the bottle has been opened, can provide enjoyment for two sometimes three weeks. For filtered LBV Ports, these typically can last up to ten to 12 days after being open, without any obvious deterioration of quality.
Colheita Port:
For younger Colheita Ports (less than 15 years old) they can last up to two weeks after being open without any obvious deterioration of quality. Older ones are best consumed within 24-72 hours.
Tawny Port - (Aged Styles):
These can last up to a month after being open without any obvious deterioration of quality, if kept in a cool-dark place. If stored at room temperature (on a back bar), two weeks is a good rule of thumb. For Restaurants - have one person keep a close eye on the aroma each day after the second week.
Ruby & Basic Tawny Ports:
Ruby and basic Tawny Ports: These typically (when stored in cool conditions) will easily last 3-4 weeks after being open without any obvious deterioration.

So why the variations: Tawny Ports (10, 20 or 30 years old) are aged in large oak barrels, where oxidation is a key part of the winemaking process - so exposure to oxygen by opening the bottle has less of an effect. A simple Tawny Port usually has a reusable cork and can last for 1-2 months after opening if kept cool.
Vintage Ports are aged in oak barrels for 2 years and then transferred to bottle where they can age for another 10-30 years (sometimes longer). They are bottled unfiltered and the sediment continues to act on it in the bottle. A Vintage Port will only last a couple days after opening.
Late Bottled Vintage (LBV) Ports are left in barrels for 4-6 years and sometimes filtered before bottling. So their life after opening is somewhere in the middle - plan on 1-2 weeks to be safe, but some can last up to a month.
Ports last longer in an open bottle than table wines, because their higher alcohol content (up to 20%) acts as a preservative. Refrigerating will extend its useful life to some extent, because refrigeration slows the process of oxidation that causes unpleasant changes in the wine, reducing its aroma and flavour.
A sign of Port losing its vitality is the ripe fruits and chocolate notes moving more to nutty notes, which will get stronger and more tired. As a rule of thumb, the older the Port wine, the faster it should be consumed.