As in quality of the liquour degenerates over time?
The alcohol in alcoholic beverages will keep the liquor from being spoiled by microbes. But the alcohol won’t keep organic compounds from breaking down into simpler substances over time, thereby ruining the flavor.
Hard liquor (e.g., vodka-strength): For practical purposes, no. If it’s stored badly and not sealed well, alcohol can evaporate, leaving you with odd-tasting water.
Cream liqueur (e.g., Bailey’s): Yes, badly. It will curdle.
Sweet, weak liqueurs (e.g., Chambord): Yes, somewhat. They’ll lose color and get more of a “dull” taste.
Wine: It varies wildly. White wine and fruit (non-grape) wine tends not to age too well, though there are exceptions. Wines such as port, tokaji, and madeira can age exceptionally well (I’ve had a blended tawny port that included wine ranging from, I dunno, 30 to 100 years old, and I’ve had madeira from the Civil War era). Red wines are all over the map.
I should note that after opening the bottle, you should generally try to finish it the same day. Some wine can go longer after opening, but you’ll probably be able to taste the difference on a day-by-day basis as it ages. I’ll try to finish a bottle of port within about a week at most; I’ll let vermouth go longer, but I keep it in the fridge sealed with a vacuum stopper and even then after a while it’ll get too oxidized and off-tasting to use.
Many years ago my favourite tipple was Pernod and water. Just a few weeks ago my parents gave me all the odds and ends from their liquor cabinet. They dated back to the days when they used to entertain and they kept a good bottle of everything - scotch, boubon, brandy, gin, rum etc many of which they personally don’t drink.
Included in the supplies was a bottle of Pernod with maybe two drinks gone. Although I haven’t had one in over twenty years it struck me as positively vile. I wondered then whether I was simply dumb and impressionable when I drank the stuff or whether it had deteriorated due to oxygenation. I may buy a Pernod and water in the club and find out.
Yup. Had this happen to me with a bottle of Bailey’s.
Not quite: the fermentation from wine (or fruit wine) to vinegar is performed by bacteria.
The oxidation of wine mentioned in other posts doesn’t involve bacteria.
To be precise, yeast is actually a member of the Fungi kingdom. When alcohol levels get too high for that particular strain of yeast, fermentation stops.
[Further hijacking] Infection can occur in beer (and probably wine) during the fermentation process, but this obviously isn’t related to how long you’ve had one of those products sitting around your house. It’s also caught early on and the batch is destroyed. Finally, infection by special bacterial strains is actually required for Belgian lambics, a particular style of beer. [/hijack done]
Nor did I say it was. That is what I was referring to when I wrote about organic compounds breaking down.
Years ago, Typo Knig and I were helping his parents sort through and discard some bottles of liquor that they had, quite literally, inherited from Typo’s grandfather. Among them were an ancient bottle of aquavit, and an equally ancient bottle of Palestinian brandy (as in, so old it predated the formation of Israel).
I think it was the aquavit that had moss growing in it (as in, floating in the liquid). So yeah, the stuff can go bad.
We didn’t sample the brandy, either.
Properly kept, an opened bottle of port will last months. Of course, it doesn’t usually last that long. 
What about Scotch? I’ve got a bottle I’ve had for like five years.
-FrL-
What kind of scotch, and how much headspace in the bottle?
Aberlour, single malt. (Is that what you’re asking?)
I don’t know from “headspace,” but if you mean how full is it, it’s about two thirds full I think.
-FrL-
Should be fine, but if you are unwilling to take the chance, for a small fee I shall dispose of it safely for you. No, really. I insist.