sentence1: Characteristics related to useful life are that  has an indeterminate or an unpredictable useful life because of the unusual manner in which it is used, improved, retired, modified, or maintained or is at a very high risk of being destroyed during use or of premature obsolescence.
sentence2: The length of the useful life of a power-tool or computer depends on how well the owner properly maintains it, follows directions in its use and does not alter the product and so forth because any such changes can lead to a shorter useful life. 