Heat, light and air are the enemies
Sunlight and heat break down the aromatic molecules in fragrance, dulling and souring the scent. UV light is especially damaging to clear bottles. Oxygen does the same once a bottle is opened, which is why a half-empty bottle ages faster than a full one.
Where to keep it
A drawer, cupboard or wardrobe shelf, somewhere cool, dark and stable in temperature. Keep fragrances in their original boxes, which block light. The worst place is the bathroom: heat and humidity from showers degrade perfume quickly despite it being the most common spot people use.
How long does perfume last?
Unopened and stored well, perfume can last 5+ years. Once opened, expect 3 to 5 years for most, though heavy oriental and woody fragrances can last longer than delicate citrus ones. Signs it's turned: a sour or sharp 'off' top note, or a noticeably darker colour.