Like Gareth, I am uneasy about ignoring a time limit. It is known that rubber (any rubber) degrades over time, so even if the car does zero miles the rubber will eventually lose its strength and flexibility over time. Consequently I would change the belt at about 70000 miles assuming a typical usage. That limit is admittedly rather arbitrary, but failure of the belt is unpredictable and will happen without warning so a line has to be drawn somewhere.