I've gotten a lot of questions from readers over the years and by far, the single largest category has to do with the math of creating a batch of limoncello with the proper proportions of ingredients. However, another big question--perhaps #2 although a distant second--concerns how long you can keep a bottle of limoncello around. I'll start with the short answer.
You should generally try to consume limoncello within 2 years of creating or opening it. Limoncello only contains 4 ingredients, 2 of which are preservatives. So, it will never “go bad” like milk would but it does lose its lemon scent and flavor over time.
Quite accidentally, I am able to give a pretty solid answer to this question from experience. A while back I moved to a new house and I happened to belatedly unpack a box, finding a partial bottle of one of my old batches.
As you can see in the photos, it is from batch #4, a batch that I reviewed on the site so I know it's about five and a half years old at the time of tasting. It hasn't been well cared for either, it was sitting in my basement at both houses. So I said "what the heck," I'll take this opportunity to see if this stuff holds up.
The verdict? This particular example held up very well! My main problem with this batch was a roughness in the flavor and all that time resting really subdued the alcohol. The limoncello was still flavorful and it is now pretty smooth as well. So, my final opinion used to be that you can rest limoncello as long as you like.
However, I've visited with some folks who keep a much more detailed history of their limoncello-making than I do (I know...strange but true) and there is a downside to resting limoncello TOO long. While it's still ok to drink, as in "not repulsive," it does become insipid after about 2 years on the shelf. It loses a lot of the lemon-y flavor it once had. That just leaves you with simple syrup and high-octane liquor.
Whether the limoncello is opened or unopened (mine above was opened) doesn't really matter. It also doesn't matter whether the limoncello is commercial or homemade because alcohol is an excellent preservative. Keeping it in the freezer the whole time, while a waste of freezer space, probably does slow the decline of the flavor as colder temperatures tend to do in most cases.
So if you leave a bottle on the shelf too long, do what I do and give it a taste. If it's still good, awesome! If it has lost it's vigor, use it in a cocktail. But whatever you do, don't let it go to waste!