Technically, nothing speaks against keeping your monitors always powered on. The power consumption in "idle mode", meaning without playing back any audio signal, is within a reasonable range. However, considering the environmental responsibility, the monitors should be turned off if they are not used for a longer period.
The F and ARTist Series and all subwoofers are equipped with a standby mode that automatically puts them to ‘sleep’ if there has not been an input signal for a certain time. They switch back to active mode as soon as an input signal is detected. How quick they react to an incoming signal depends on its energy.