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The VW maintenance manual is 100,000 miles. However, it has been my experience that they many times do not last that long. In the shop we always recommend that you replace the belt at 70,000 miles. I have owned VWs for many years as my personal vehicles, I have had TDIs and always to be safe replace them between 70,000 - 75,000 miles or if they are older than 6 years regardless of mileage.
The belt is a rubber item and like any rubber tires, belts etc, they should be replaced anytime they are over 6 years old. So if you have a vehicle that is not driven much but has reached that age, I also recommend you replace the belt. Another suggestion is when you have the belt replaced to replace the water pump at the same time. We have seen a high fail rate on other motors for the water pump impeller, and the TDIs are the same impeller. The water pump is driven by the timing belt