Normally, there is no specified interval on changing timing chains. They are supposed to be a "lifetime" part.
Concern yourself with it when there is evidence of a problem: rattly noise, tension not being applied, etc. At that time, a proper inspection and diagnosis will reveal what needs to be replaced.
It is possible that the service manual gives a tolerance for the 20-link (or similar) length of the chain, in which case, inspection is possible.
It is also possible that by the time the chain or tensioner ever gives trouble, the cost of properly diagnosing and repairing it may be worth more than what the vehicle is worth at the time.