You generally need a new belt when either 1) its thickness is less than the minimum recommended thickness or 2) when its starting to look a bit ordinary (ie cracks, weathering etc appearing on it). But as everyone will tell you, its better to replace it before it breaks...
When we have to replace belts before they are expected on the service schedule, we'll get approval from the customer first and let them know why we recommend it. Then we'll give them the worn belt when they come to pick up the scooter and show it compared to a new belt. Its not hard to reassure someone what you are doing and why. Good brands have decent service manuals that go into detail as to what needs to be done at each service. Anything that is done outside of those schedules should be able to be fully explained by the mechanic. If they can't tell you what should be done before the service, suggest you go elsewhere.
But some scooters belts will last well in excess of 10,000km. Depends on the scooter, the quality of the belt, the weight being driven around, and how its being ridden. But Montego's always seemed to go through them quicker than you'd expect, so I suspect that all the other clones are no different.