How do you know when your Keurig coffeemaker needs descaling?

All models of Keurig coffeemakers come equipped with a descale error light to alert users when the machine requires descaling. However, the official Keurig website recommends descaling a Keurig coffeemaker every two months, before the descale error light appears.

To descale a Keurig coffeemaker, pour a combination of descaling solution and fresh water into the machine's water reservoir. Press brew, and allow the liquid to pass through the interior of the coffeemaker. Repeat this process once more with descaling solution and an additional three times with fresh water only. If a residual taste remains, repeat the process with fresh water until all traces of the descaling solution are erased.