Water kefir grains can live indefinitely. They are hardy, alive micro-organisms that are in a constant state of reproduction. As long as you follow the Happy Gut Method for brewing and periodically give them rest by putting them into dormancy, they will reproduce and ferment indefinitely.
Some may get weaker over time for one reason or another (neglected, frozen, etc) but for the most part they will continue to do their job.
The only way that your water kefir grains will begin to disintegrate is if they are touched by metal, by being exposed to too high of mineral content or by long periods of neglect.
Here are some tips to keep them thriving: