There is no single correct answer to this question. As spark plugs age, they lose their sharp edges as material from the center and ground electrodes is slowly eroded away.
As the gap between these two points grows, the voltage required to bridge the gap increases. Even the best ignition systems will be stressed to supply enough voltage to completely burn the fuel.
Replacing worn out spark plugs with new ones effectively restores the ignition system's efficiency. Misfires are reduced, power is restored, economy of operation is enhanced and emissions are reduced.
The best guide is the manufacturer's recommendation for your vehicle, as this particular service varies from brand to brand and model to model.
In the absence of this information or in conjunction with it, you can rely on the advice of a mechanic who is familiar with your type of vehicle.