Most belts seem to need replacing around every 60,000 to 70,000 miles of wear. You should be able to find this information in the service manual for the car.
This is only a rough estimate as some belts will snap unexpectedly before this and some will still be in good condition after 100,000 miles of wear.
Replace if you are in doubt
If in doubt, you should always replace your belt rather than waiting to see if it lasts. The cost of having a new timing belt fitted will be much lower than the cost of repairing a damaged engine if the belt snaps unexpectedly.
The best way to check the condition of your timing belt is by looking at it. By opening the bonnet and removing the covers protecting the belt, you should be able to make a visual assessment of whether the belt needs changing or not.
When you look down at the belt, if the outer surface is visibly worn then it is probably time to change the belt. If you can see thinner patches in some areas or if there are small cracks in the synthetic rubber then you definitely need to think about replacing the belt soon.
If it snaps entirely
Timing belts do not gradually stop working over a period of time. They are such a vital part of the car’s engine that it doesn’t matter whether the belt snaps entirely or whether a few of its teeth are stripped from the belt’s inner surface, the result will be the same: the car will not work until the belt is replaced. If a couple of teeth are stripped, the belt will not be able to grip onto the heads of the camshaft and the crankshaft properly, so the belt will either slip or stop transferring the power from one part of the engine to another.
Change at regular intervals
To avoid your timing belt snapping unexpectedly, the best thing to do is to have it changed at regular intervals. Having your belt changed for every 60,000 miles that you drive should prevent the belt from wearing out completely whilst you are driving. If you are going to be travelling on a long journey, it is a good idea to get a mechanic to have a look at your timing belt before you set off.