There is no specific time to change spark plug wires but an ideal time would be when fuel is being left unburned because there is not enough voltage to burn the fuel.
As spark plug wires age, they lose their sharp edges, and ground electrodes become eroded. The wider the gap between the two means that greater voltage is required. After straining the engine to supply enough voltage, spark plug wires will need to be replaced because some fuel will be left unburned. Two sources of valuable information for changing spark plug wires are the car manufacturer's recommendation and also the advice of a trusted mechanic.