You won't know until it's too late. Stick to what Haynes state in their manual for the car. In the past manufacturers have been "optomistic" with their replacement intervals. The replacement cost is measured in hundreds of pounds and the damage consequent on failure in thousands. I've been there! Peter Edward
theres really no way in knowing if it will go or not,but i would take notice of when your dealer says and replace it,i had one go and it cost me a new engine.
My advice is: replace the cam belt KIT!
we got a used car , a mondeo, garage said the belt had been done after 70 000 miles BUT when a mech mate looked it over, there were cracks in the pulleys the belt went round! that would have failed within months - so dont muck about, get the lot changed to prevent catastrophic failure.