Life expectancy: This part of the specification will state how long the product should remain in working order provided the customer gives reasonable care and maintenance. Also take into account technological advances and ongoing improvements that would make the current model obsolete. The design process is to be considered an ongoing event. If we consider each improvement as a new product then we can see that the initial and subsequent products will have a distinct lifetime. The other factor to keep in mind whenever considering life expectancy is the individual life expectancy of each component. It would be senseless to make one part capable of lasting decades if another component fails within a couple years (unless you plan to make replacement parts available).