every 100,000km or 7 years is the general rule. The rubber degrades over time no matter how low kilometres you do. Most people here have probably had their skylines reach the 100,000km milestone with the factory fitted timing belt, 89 GTR for example that changed it this year.. that's coming 20 years old.
Timing belt snaps and the valves smash into the piston, and takes a a piston or 2 with it.
Tensioner/idler pulley seizes, the belt is locked in place and valves are smashed into the piston, and a takes a piston or 2 with it.