Don't know what the record for original timing belts lasting is but I just bought a 1997 960 with 145,000 and had the timing belt changed for the first time. I confirmed with the previous owner and he checked with the original owner. My understanding is that it should be changed every 70,000 miles, which seems pretty often.
The previous owner didn't know a thing about changing it. I felt I was on borrowed time the week I drove it with the original belt.