How often do you need to change the belt on mk5 golf gt tdi?

I believe it is every 80,000 miles or every four years, which ever comes first.

For example if your vehicle is coming up to 8 years old with 35,000 on the clock it will still need a cam belt change on time not mileage. It should have had one at 4 years old and then 8 years. So in effect on it's 8th birthday it should be on it's 3rd belt.

It is best not to stretch any of these intervals, and if in doubt change the belt, tensioner etc, c/w water pump.

Hope this helps.