Most life-forms can only function at low levels of alcohol. One reason is osmosis, which we discussed in a previous section. But the big reason is that alcohol is really just poisonous. Even yeasts, which make alcohol as a product of digesting sugars, can only stand so much. That's why wine has only so much alcohol in it. When the yeasts stop fermenting the sugars into alcohol, you're done. To get higher concentrations of alcohol, you need to distill it, which is beyond the scope of this site. Rest assured that any alcoholic beverage with more than 15% alcohol is quite safe from biological contamination.
Anyway, you must wonder what, then, does cause liqueurs to spoil. And the answer to that is air. Or, more specifically, oxygen. Oxidation is the bugaboo of most everything organic, and it's the main bugaboo of alcohols. Oxidation is the process that turns fruit brown - it breaks large molecules into smaller ones, complex flavors into simpler ones, good liqueurs into stale, insipid or even sour ones. And it's something you want to avoid if you can.
When a liqueur spoils due to oxidation, it does what you would expect a fruit to do. It turns brown, the sugars break down, and many suspended solids stick together and form a clump at the bottom. Often, the colors and flavors will separate considerably, leaving you with a dark, cloudy bottom half and a sickly yellow top half. It will never hurt you to open up the bottle and taste it, but don't expect much. Some fruits rich in pectin may also gel. Sugars may fuse with other chemicals to produce glycerin, making the liqueur thicker anyway. And the pectin inside may form isolated clumps of odd, alcoholic jelly floating in your bottle. It happens to my nectarine after two or three years, though I've never had the guts to taste the jelly. But a general rule of thumb is that when your liqueurs start to fall apart or change state, its time to toss them and start fresh.
Wine corks are standard-sized, and though they can vary in quality, you should be able to buy good, supple ones for something around $0.35 apiece. They are well worth the investment. Most unusual bottles come with brittle, porous, tapered corks. They are pretty much useless for actually sealing bottles. Beyond the obvious hazards of stiff, brittle and porous material, the tapering angle is probably the most serious concern. What makes wine corks good is that they're cylindrical - straight from top to bottom. Compress a cork into the neck of a bottle, and it's an inch and a half of solid seal, in complete contact with the glass. If there's a bubble, a warp, or a crease 1/4" down the neck, it's still sealed for the remaining inch. Even if there are many slight imperfections, as long as they don't form a continuous chain, you're still sealed. But tapered corks really only grab the neck at a line, or at best a narrow ring. If there's an imperfection in that narrow band, you're sunk.
As an aside, I occasionally see, and use, "Grolsch"-style bottles - bottles with a rubber ring and latch much like the jars I use for steeping. Although you might think these would be just as good as their jars, they seldom seal as tightly as jars. I find they leak - faster than good corks, slower than bad. They'll last fine for months at a time, but they're no good for serious, long-term storage.
It's worth saying, of course, that any container is fine for short-term storage. If you're bringing it to a party to be consumed that night, you can pour it in an open-air bucket. But if you want to store it for two or three years, get the best storage containers you can. I could say it's worth the investment, but good wine bottles are actually considerably cheaper than fancy bottles. So think about how the liqueur is going to be treated when you're selecting bottles. I usually go for a mix of fancy but unreliable bottles and boring but solid ones, with the expectation that the fancy bottles are going to parties that year, and the boring ones are to be kept in personal liquor cabinets.
The proper way of handling corks, I've been told, is to stand the bottle upright for a few days to let the pressure equalize a little, and then set the bottle on its side. Leave it there until you're ready to open it. If you get slight leakage, wipe it off and leave it - it's just the higher-pressure environment inside the neck trying to equalize a little more. It'll finish. Actually, my SO and I joke that we prefer to bottle in a rainstorm - the air pressure is lower then, so we'll get less leakage in the long run.
Other than cork husbandry, keep your liqueurs in a cool, dark and relatively dry place. Heat and light can both cause oxidation-like effects even without the air, so if you shelter your liqueurs you will have them longer. Dryness keeps things like mold and mildew from forming on the outside of your corks. This may be just unsightly, but that can put a lot of people off their palates thinking about it. And it's always possible that the mold can degrade the cork and cause it to fail.
Some people suggest using what are called capsules on the necks of your bottles. These are wax (or foil) caps that sit on the neck of the bottle. At first, you might think these are great to keep off the occasional mold spore, and even a way to supplement the effectiveness of corks. But wax is awfully porous. Porous enough that it won't help at all against oxidation, and over a few years it won't even keep the molds out. If you want to use wax capsules, put them on last, right before you present the bottle to its recipients.
I freely admit I may be wrong, and maybe it's all a matter of how good your bottles are, but my experience hasn't been great for long-term handling. Then again, I am lazy...
My general rule of thumb is: if the bottle is never opened, expect three years out of it. If it's opened once, or seldomly, expect a year or so. If it's opened lots, expect that you'll finish it soon anyway. There is so much fresh oxygen hitting it that it will be bad in a couple of months, so you may as well drink it while you can.
But commercial liqueurs can last considerably longer. They have commercial preservatives and antioxidants to use. They've got a vested interest in increasing the life expectancy of their brews, and the financial wherewithal to make it work. So if you're here wondering whether your 40-year-old Benedictine is still good, I would bet it is. And since you're not going to poison yourself by trying, I would recommend just opening it up and seeing.
There is a second class of liqueurs that bears some consideration - cream liqueurs and egg liqueurs. Despite alcohol's great disinfectant properties, every liqueur book I've ever read cautions people against trying to keep cream or egg liqueurs for more than a few weeks to a month, and then only in the refrigerator. I'm forced to make the same recommendation. Even with the alcohol, treat it like fresh milk or fresh eggs. If you would throw out milk that had been in your fridge for that long, you should throw out your liqueurs. So these should be made immediately before they are to be consumed. It is perhaps possible to keep cream liqueurs longer, but in order to experimentally determine how long they can last, I would be required to, on at least one occasion, consume spoiled cream or egg liqueur. And that I have no strong urge to do.
Again, commercial liqueurs last considerably longer. Eierlikör and Bailey's Irish Cream both have much longer shelf life than four weeks. But it is because they know what they're doing that they can get long shelf lives. As long as we're just puttering in our kitchens, we should be a good bit more careful.