For a Keurig K Slim coffee maker, Keurig recommends descaling every 3 months or when the descale light starts flashing. Follow the instructions on the descaling solution that you are using. Some recommend rinsing with 3 full reservoirs of water.
Check out more home improvement tips on our weekly podcast
https://itunes.apple.com/us/podcast/fix-it-home-improvement/id880903087?mt=2