Once I've opened a bottle of hard liquor, is it still good indefinitely?

Everyone seems to agree that an opened bottle of these liquors is good for quite a while. But will it ever go bad? This is where things get vague. One expert says the "essence will decline as time goes on." Yahoo Answerer assures readers that hard liquor will keep "for years" in a cool, dry spot. Colleen Graham at About.com can't get any more specific than this: "Opened bottles...are exposed to some air over years and may (emphasis hers) lose some of their finer qualities, which may or may not be noticeable." And Bacardi said pretty much the same. To summarize: If you happen upon an opened bottle of gin that you stowed away years ago and forgot about, it might not taste as good as it once did.
 
We wish we could be more specific than that, but we suspect the lack of data is due to the impracticality of the research; simply not enough people are willing to keep an opened bottle of liquor around for several years just for the purposes of a taste test. But again, at the very least, there is no evidence that the liquor would "go bad" enough to be any sort of hazard, nor could we even find anecdotal evidence that the taste is significantly affected after several years.
 
 
Source(s):
Cocktails.About.com - "Liquor Storage Myths"
"Does Tough Liquor Move About Bleak?" - Food-FAQ.com