12V input and 12V zener = 0V left across the 100R resistor. Current through zener is zero. The zener cant conduct because you don't have enough voltage.
24V input and 12V zener = 12V left across the 100R resistor, so current is 12V/100R = 120mA max
However the real world value will be less because the light sensor, or whatever that thing is, will draw some of the current away from the zener.