UV Bulb Life Based on Normal Usage
According to General Electric the average UV light has an unlimited shelf life as long as it is kept in a dry, dark area at a moderate temperature.
UV Bulb Life For Aquariums
When using UV bulbs in aquariums, the shelf life of the bulb does come into play in terms of how effective the light is on thwarting algae and scum. According to American Aquarium, UV bulbs for aquariums have a shelf life of up to three years maximum, as dust will hinder the bulb's effectiveness in an aquarium more so than dust would hinder typical use.
Germicidal UV Bulb Life
UV bulbs are also being used for food processing, at medical facilities, in clean rooms and in phototherapy. This UV bulb, a germicidal bulb, has a shorter life than generic UV bulbs or UV bulbs for aquariums. FoodProcessingTechnology.com recommends that any UV bulbs stored longer than six months on the shelf be replaced.