The sheet
@Loz posted shows 120k miles.... but doesn't state a age...
Other vw sources as per
@Scruffy post above state 4years.
Most other modern diesel engines are the same.
4years due to natural fatigue, shelf life, heat cycles, stretching etc etc
So let's say its 120k miles or 4 years..
And it cheaper to replace a belt every 4 years than ot is to replace an engine.
(You can call VW UK and quote your reg number and get the official word from them) - it would be interesting to see what they say.