The purpose of using a de-scaling agent is to maintain that the milk frother is clean. However, you should consider the hardness of the water and the frequency of use. The new milk frothers have an inbuilt tester that will determine the hardness of the water. Also, most of them feature descaling alerts that help in keeping the machine in optimal condition. If your device is non-automatic, you can descale it once per month or when you note a poor frothing performance.