You keep hearing that because there are a lot of prudent people out there that would prefer to do this work more often than is stipulated by LR, to avoid a costly breakdown.
The change period is measured in miles and age. Remember these belts are a composite of rubber and other materials - rubber perishes over time. Some cars never get their belts changed, others get done at yearly intervals.
If you can do the work yourself, you might considor it a sensible precaution to do the change more often than the 72k/6 years reccomended. I've heard of some owners changing the belts every year as part of an annual service !!!