For people, animals and plants, lifespan refers to the length of life, according to the Oxford Dictionaries. For equipment, machines, appliances and other products, lifespan describes the length of time for which the item is functional.
The phrase "expected lifespan" describes the length of time that the average human lives. As health and science evolves, human lifespans get longer, according to Scientific American. The expected lifespan, or useful life of a product, is one of the factors evaluated by potential buyers. A durable product that lasts longer offers more value than a product that is expensive and has a relatively short lifespan.