The longest I've ever had a liqueur last was three years, even with good corking. Well, you'll remember that much of your liqueur creation, especially filtration, was done in the presence of open air. That's a lot of oxygen dissolving into your liqueur. In time, it will kill your liqueur from the inside. And for home use, it's really impractical to try to work without exposure to air, especially that dastardly filtration step.
Wine makers do much of their work in plastic tubes and glass carboys.
And their filters are often closed pumps. But with liqueurs, the stuff we're filtering is much larger, and would overwhelm wine-makers' pumps. If I had to guess, I would say that no home-made liqueur would reasonably last more than 5 years, even with antioxidants like potassium sorbate added. So all you can pass down to your grandchildren are the recipes. If it's opened once, or seldomly, expect a year or so.