The design life of a component or product is the period of time during which the item is excected by its designers to work within its specified parameters; in other words, the life expectancy of the item.
The design life of components often is denominated as a mean time between failure (MTBF), which tells the user that they may expect the average component to last for the specified time. Note, though, that an MTBF rating is no guarantee that any single example of the product or component will not fail before the mean time specified.
Many products and components employ design-life as one factor of their differentiation from competing products and components. A disposable camera is designed to withstand a short life, whilst an expensive single-lens reflex camera can be expected to have a design life measured in years or decades. (Clearly in this example there are other differentiators).
In general, entry-level products - those at the lowest end of the price range fulfilling a certain specification - will tend to have shorter design lives than more expensive products fulfilling the same specification, since there are savings to be made in cutting designs to the bone, or, conversely, costs to be passed onto the customer in over-engineering to provide a safe margin leading to an increased working life. This economic truism leads to the phenomenon of products designed (or appearing to be designed) to last only so long as their warrenty period.
Design life is related to but distinct from the concept of built-in obsolescence. The latter is the a somewhatmore nebulous notion that products are designed so as to become obsolete - at least in the eyes of the user - within a timespan lower than their design life. Two classic examples here are computers and cars; the first becoming obsolete as a result of the development of processors, operating system and application evolution; the second becoming 'obsolete' due to little more than, after a year or so, not being "this year's design".