The Ad Blue, or diesel exhaust fluid as most people know it, is part of your initial scheduled maintenance and should last as much as the engine oil. However, consumption of the fluid can vary due to climate conditions and your driving style.
If you're low, a special light will flash on your dashboard. Be warned that if you do run out of the stuff, the all-mighty Passat will probably not start. The tank is sutured on the right side of your tank, so you can buy a bottle of Ad Blue from your dealer and fill it up yourself if needed. The bottle has screws into place and you have to press it down to fill the tank.
Ad Blue releases ammonium into your catalyst, reducing NOx emissions into oxygen and water.