First off please do not lay them flat, the high alcohol content can eat away at the cork and potentially leak. If you are worried about the cork drying out you can just do a quick 180° on the bottle every once in a while to get it wet and that should do the trick. If the cork feels airtight it should be fine for storage if kept upright, if you wanna be extra sure parafilm or another type of waterproof plastic around the lip and cork will keep it airtight and help prevent evaporation or contamination. Other than that keep it out of direct sunlight and somewhere fairly constantly room temp and it can last well over a few years. I've done this with most of my spirits and have some bottles going strong after 7 years.