Does hard liquor keep forever? I drink mixed drinks infrequently, so it takes me a while to go through a bottle of rum, whisky, or vodka.  Thus far I haven't noticed a problem with, say, a two-year-old (opened) bottle of any of these, but on the other hand, if they were degrading slowly I might not notice.  Does the flavor of hard liquors change over time after the bottle has been opened?  (Assume it is tightly recapped after use.)  And are there any safety considerations?
I have never head of safety concerns, especially if the bottle is properly recapped, but what often is an issue is oxidization, and exposure to sunlight.

It happens much more slowly to liquors than to things like foods or wines, but every time you open the bottle you let oxygen get in, especially so the first opening, which will affect it though its really only slightly. The sunlight has similar affects on the spirits, to a similar magnitude (i.e. not very much).

Vodka for example should be fine nearly indefinitely if properly kept, but whiskeys, gins, tequilas, rums, and the more complex liquors will degrade (again, only slightly and slowly). I've heard people say they can taste changes in 3-4 months, others years, but even if you have a bottle around for a year or two I really wouldn't have any concerns (I'm kinda skeptical of the 3-4 month people)

I've had whiskeys and rums left around for 4-5 years after opening. They weren't great spirits themselves, but there was nothing unpalatable about them.

If you are concerned, I would just keep your spirits in a cool, dry, dark space and then you probably don't have to give it a second thought.