Shelf life of homemade limoncello?

I made lemoncello a year ago using Absolute 80 proof. It's been refrigerated ever since. I put some in a new bottle to give as a gift, and after about 3 hours on the counter, the bottle broke/exploded on the counter and I had a sticky lemon mess all over the kitchen. Any idea what would of caused the bottle to break suddenly?

I make it all the time, using cheap vodka mixed 50/50 with syrup (syrup is 3/2 water to sugar by volume) It lasts a very long time, even unrefrigerated. If you used everclear (151 or 190 proof) it is unlikely to freeze in your home freezer. In mine, my mix gets slushy, but I start with 80 proof, so I end up with about 40 proof ( i.e. 20% alcohol) I agree with dynamo that too much alcohol can overwhelm the lemon flavor.

I have kept a bottle in the freezer for years...it doesn't freeze. The alcohol prevents freezing. However if you have added a lot of simple syrup (sugar and water) sometimes it becomes slushy. If you used everclear which is pretty high in alcohol you will have no problem with freezing but I have found the extreme high alcohol can overwhelm the lemon flavor with time. I tried it (moonshine from Tennessee) and went back to using vodka when making spirits with fruit like lemons and oranges.

It will most likely last longer than you will. Everclear is 100 proof. It's a natural preservative and won't freeze. It's certainly going to inhibit the growth of any spoiling agents. Compare it to preserved lemons which are held at room temperature.

My bottle lives in the freezer for the same reason many people store vodka that way. It doesn't even come close to freezing although I suppose YMMV depending upon the recipe (but it's easy enough to test). The refrigerator is fine too. It won't take on stray flavors anymore than your milk does. As for lifespan, I suspect you'll consume it long before it degrades, certainly if it's stored < 32F.

Freezing it should be fine. I've frozen homemade and store bought limoncello with no problem. I would just make sure that the syrup is fully dissolved in the everclear and that it is a true solution. If it is a solution, it will freeze just slightly but for the most part will just get very cold. I would make sure that there is enough room for expansion in the bottle, just in case. So put a bottle that is about 3/4 full (or less). Personally, I don't like to store liquor in the fridge because it takes on odours/flavors. Limoncello with left-over lasagna flavor is not an appealing thought.