How many years does it take for Apple xsmax to get 79% power?

About two years.

The battery life of Apple iPhone will drop below 80% in about two years.

Apple's battery health has reached 79%, which usually lasts for about 3 to 6 months, because if the battery health is too low, it will affect the performance of the mobile phone. The speed at which the battery health index drops is closely related to the personal habit of using mobile phones.