This depends on the specific ingredients in the formula and environmental factors. Perfumes and fragrances can keep their olfactory integrity for an average of two to five years but an almost empty bottle will oxidize faster than a full one because it contains more air. Oxygen, heat and light are conducive to the ingredients’ oxidation which alters the odor of the fragrance, especially the top notes, which are the first ones to go bad. There are no health risks associated with wearing an oxidized fragrance but it will smell funny, especially the first minutes!