I own a Mercedes-Benz CLK 320 as well with the FSS system. Well if the battery is disconnected, once the car is put back in use the oil flows through again and the system at that point takes a measure of oil quality and the computer will adjust accordingly. I dont think it has a timer.

If the car has been stored for about 6 months, once the car is started and oil circulating only then is when the system takes a reading of oil quality. So as soon as the car is running again the service date will adjust accordingly i dont know how quickly the system takes to calculate this i'm assuming not long at all. I dont have all FSS answers however i'm very familiar with the operation.