It depends in the engine pal. Some manufacturers range from 40,000 miles 4 years to over 100,000 miles 10 years. From my previous seat diesel it was 6 years without fail regardless of mileage (unless you had gone over the mileage first). It's not like an air filter when you can say nah f**k it until next time the belt is under load and the rubber perishes as well.
Like above look into this and also is there any evidence it was done at that other place?