Depending on the make, model and how you drive a car, the AdBlue range can vary quite substantially. The Touareg has two different sizes of AdBlue tank - 12 and 24-litres. On average (but it does depend on your driving style) a car will use a litre of AdBlue every 800-1000 miles, so assuming your car has a 24-litre tank then the mileage you mention is within range. Your car should first alert you that it needs a top up within 1500 miles of becoming empty.