I replaced mine at 100,000 miles. It is quite a pricey maintenance issue. My VW dealer gave me an estimate of $1400. I found a local import mechanic who replaced that plus the water pump for around half the dealer estimate. I would have the water pump replaced at the same time since it has to be removed anyway.
The standard replacement time for a water pump and timing belt is every 80,000 to every 100,000 miles. If you are not the first owner of this vehicle and you do not know whether this has been done before, than there should be a sticker under the hood verifying the timing belt change.