This function is available in different settings. Firstly, when you turn on an appliance, you can also set a timer off interval. You set this each time the heater is turned on and it will then countdown to the interval duration and automatically switch the appliance off. Set this function by clicking the appliance name to go to the detailed screen and setting the timer off internal at the bottom of the screen.
Alternatively, you can set the timer off delay using schedules. This feature is only available to WiFi connected appliances.