Liquor doesn't have an expiration date like milk, nor a "best by" on the label like some beers do. That said, high-proof spirits can go bad once the seal on the bottle has been broken. Their drinkability is a matter of taste more than one of food safety, as it would be with a perishable beverage that has a time stamp on it.
The Oxidation Blues
When you find a forgotten bottle of liquor at the back of the liquor cabinet that's already been opened, you may wonder whether or not it's safe to drink. If the bottle is more than one-third empty and has been sitting for years, it may taste bad -- or not as it's supposed to taste -- because of oxidation. Simply put, too much air is affecting the spirit. It's almost as if the liquor is rusting, even though it doesn't contain any actual rust in it. The emptier the bottle, the more oxidation goes to work on the spirit because of more air in the bottle. In short, it's not dangerous to drink, but if you have a bottle that's been sitting two-thirds empty for more than a year, no need to chuck it. Instead, invite some friends over and finish it off before the taste starts to turn.
For spirit snobs, somewhere between six to eight months is the cut-off point for whether an open bottle of liquor has retained its original and desired taste. In fact, this is the industry standard, according to spirit sommelier Ethan Kelley in an interview with theKitchn.com. Also, unlike wine, which is commonly stored sideways, liquor should be stored upright so that the spirit doesn't come in contact with anything but the glass of the bottle. Store your bottles at room temperature and away from light.
Sealed vs. Opened
Unlike an open bottle, a properly sealed bottle of liquor can keep indefinitely without the taste being affected too much. It's still important to store a sealed bottle away from light and in a place where the temperature doesn't fluctuate, just to be safe. In open bottles, alcohol can slowly evaporate over time. Never hide a prized bottle of liquor in a place like the attic, whether it's open or not, as temperature extremes could cause it to lose flavor and punch.
Liquor vs. Liqueur
One of the reasons liquor keeps on the shelf for so long is because it's a higher-proof spirit without any additives. Liquor is a distilled spirit of 80 proof or higher, meaning it contains 40 percent or more alcohol by volume. Liqueurs, on the other hand, are lower-proof beverages usually containing some kind of fruit, flavored sweeteners or cream, and they degrade in flavor faster than liquor. Many liqueurs can taste spoiled within a year after opening. If it smells bad, it probably is, so chuck it.