Timing belts and chains should be changed before they break. It is bloody expensive to repair the damage if one breaks at speed, because the pistons hit and bend the valves, which sometimes make holes in the pistons.
As you can't tell when they will go, the only thing that you can do is stick to the manual recommendations.
Belts are the modern way, and seem to be less reliable than chains, but chains wear the cogs which drive them.

Stick to the instructions, I do.
Or buy brand-new cars.