Because of the added distilled spirit, once opened, ports keep up to a year if they are stored in a cool, dry area with an airtight cork. They will lose some of their freshness after a few months, but no worries.
Not so long ago my mother served me a glass of ruby port from a bottle opened 2 years previously and stored in the kitchen cupboard - not great but by no means vinegar - I wonder if we are all missing a trick
The notion that 'port doesn't keep' isn't borne out by my experience.
Not only does it keep much better than many assert, I have repeatedly found that VP and other unfiltered wines improve significantly after a day or so in decanter.
I have also encountered bottles of tawny that have lain half drunk for a long period and are still in good shape.
In my experience, port (many sorts) can keep for months and months. Sure they loose a lot. A vintage character "decrease" to Ruby-level etc. But if you're just having a glass of port with your coffee in the evening (not meditating over it), most port keeps for months and months.
What I lack in size I make up for in obnoxiousness.
