There is no guarantee on how long a WH will last...there is some variation unit to unit even on the same production line on the same day. Then, take into accout the water, use, and maintenance, and the variation increases.
Some home owners associations (say in a condo or rental) replace them on a regular basis to ensure they are unlikely to fail, causing damage to multiple units. This ends up being cost effective, but could be shortchanging people as the units could likely last longer. In your home, it depends. If it was installed with a pan and drain so it wouldn't create damage when it fails, then you may just go until it does, then replace. If it sits in an unfinished basement where the water wouldn't create a problem, you might leave it as well. But, if you don't want damage, or to be without hot water at an inconvenient time, replace it when it gets old. How old, is up to you and would somewhat depend on maintenance and local experience with the brand.