The light switch is part of the Lamp Control Module. The Lamp Control Module is on the CAN network. When the network is active the modules on it will be awake. As long as the lamp Control Module is awake then the lights on the switch panel will be lit. The network will be awake for about 15 mins after the car is locked or about 30 mins if it is left undisturbed but unlocked and with all doors, boot etc closed. If you open a door or boot the network will wake up. If the light switch is staying lit longer than it should then something is keeping the CAN awake. This is usually a module fault. Diagnose it by disconnection the modules and see if the draw drops but remember that everytime you open a door or disconnect the battery you must wait for the CAN to go to sleep again before you can measure the real draw.