The handbook will tell you. I've seen various figures quoted.. from 60,000 to 72,000 for my 1.9 JTD. It seems to depend on the climate and the precise model year.
If it was my old beast and I had no idea at all, I'd go for the safe option of 60k or every 5 years.
Also change the tensioner, pulleys and the water pump.
Ralf S.