Port..how long does it last?

[RAD]

Let's finally put this one to rest, and see if we can't take it from anecdotal accounts to actual fact. So the actual issue in question is:

Obviously, the higher the alcohol level, the longer it can remain open. Wine is typically 11-14% alcohol, and obviously doesn't last more than a couple of days. Spirits, on the other hand, can be 40-50% or more. Port is obviously closer to wine in terms of alcoholic content. I've no experience with vintage port; mostly tawnies. Are tawnies higher in alcohol? The bottle of port I currently have open is Joseph Phelps Le Mistral light grenache port, 16.0% alcohol, 11.5% residual sugar.

I'm not a chemist. At what percent alcohol is an alcoholic beverage "over the line," and more or less safely able to remain open?

RAD