My experience has been that belts are usually 60,000 mile OR 6 years, whichever comes first (replace 60,000 and 6 with whatever is relevant to the particular model), but rubber won't last forever. I've just replaced mine at 50,000 miles, but 13 years old. I'd certainly be worried if I had one over 10 years old that hadn't been done, no matter what the mileage.

Nick