Light, heat and air ruin perfume. Fragrance isn't designed to be hoarded — it's made to be worn and enjoyed in the first few years of its life. The longer you hold on to a bottle, the more the original composition starts to fade and change.
That said, if properly stored, some fragrances can last for years, decades, even longer. Scientists are actually working on reformulating the dregs of a 3,500-year-old flacon of perfume found in Egypt. But to avoid having to call in the fragrance forensic team, keep your precious elixirs in a cool, dark place — away from scent-eating gremlins.