There is not a standard change interval for cam-belts, it varies with manufacturer and model, but if a cam-belt breaks it can cause immense damage to your engine amounting to hundreds of pounds in repairs.
Depending on the model and the manufacturer, the recommended interval can be anywhere between 30,000 miles and 100,000 miles. There will often be a time-based interval specified too e.g. 60,000 miles or five years, whichever is the sooner.
Check with a franchise dealer for the model concerned to establish the requirement for your particular car – make, model, engine number and model year. A car that is used in short journeys will often create more wear than one used on long journeys as stopping and starting are the primary cause.
Failure of the cambelt will in most cases result in serious damage to many other components in the engine – a replacement engine may well be required!
It does no harm to renew the belt early and you should do so if you buy a used car and are in any doubt if the cambelt has been renewed according to the maker’s schedule.