For people, animals and plants, life span refers to the length of life, according to the Oxford Dictionaries. For equipment, machines, appliances and other products, life span describes the length of time for which the item is functional.
The phrase "expected life span" describes the length of time that the average human lives. As health and science evolves, human life spans get longer, according Scientific American. The expected life span, 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 life span.