The Passat W8 seems to have a massive water tank and the car status computer brings up "Fault 1/1 - Topup Washers" when there is still a good weeks worth left. It than continues to beep at you and flash the message up everytime you close the drivers door.
My FreeLander on the other hand, has no such cleverness. You also can't let it run out. For some reason when the water gets low, the pressure drops so much the water barely reaches the windscreen. Stupid car. You find yourself driving quicker so the wind takes the water up the screen.