Storing, Handling and Spoilage of Liqueurs
"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."