My 1991 S4 --- according to the history / service records I have, the Cam belt was last replaced in 2007 and 30,000 miles ago. No warning light showing. I had a look and the belt appears as newish.
I am probably going to get a bunch of desperate warnings from some Members. -- I might be told to leave the club ! I am NOT talking about track or street-race cars or cars left in a Alaskan barn for 20 years or stored in Dubai. For many 928 owners and 'mature' users in temperate climes :: My thinking is that in the 1980's when the 928 was new the drivers and the cars were young & enthusiastic, no speed cameras/ trackers, fewer speed restrictions; so 4,000 ++ r/m sustained and high annual mileages common. Belts failed. Now anything much over 3,000 r/m sustained risks your licence in most jurisdictions and many owners do low annual mileages.
And belt materials are utterly different since the late 1990s. ( along with hoses that once failed every 18 months now last for 20 and more years).
better belt materials, mature users, speed cameras, low annual mileage, not much high rev use: fixed interval belt change may no longer be appropriate.
In Europe fuel at about 7 U$D or more per each US Gallon is also an incentive to use your engine gently.
BUT. If you see a request from me for anyone with a replacement engine --- please be kind.
Peter

 

My take on the cambelt is that the biggest "belt killer" is most likely lack of use wherein the rubber molecules tend to stiffen up through lack of flexure. Running the motor for 5 minute every couple of weeks would probably mitigate that risk.

If one had the belt on for 10+ years then it owes the owner nothing and time to go in there and inspect everything. Given a new belt from Roger runs at $30 plus a tensioner boot/clamp, tensioner pivot bush and bearing costs about $100 in total it is really a no brainer for a DIY effort or if carried out in a shop