The spark plug wire set (or ignition wire set) is part of the secondary ignition cables. The ignition coil wires send the spark from the ignition coil to the distributor cap and the spark plug wires then send the spark from the distributor cap to the spark plugs. A spark plug wire set replacement is needed when the spark plug wire does not deliver the spark to the spark plugs. This failure usually happens when the insulation that surrounds the wire beaks down.
A spark plug wire set replacement is usually performed around 100,000 miles, but it is best to refer to your manufacturer's recommendation on when to replace the spark plug wires. To replace a spark plug wire, the wire is removed from the distributor cap at one end and from the spark plug at the other end, then replaced. Make sure that the ignition wire set is routed correctly, because if it is not then the wires wither don't come into contact with the engine components or they rest on the hot exhaust manifold.
Since a spark plug wire replacement is part of the regular maintenance of your vehicle, you would be best advised to follow the factory recommended vehicle maintenance schedule for how frequently to change the spark plug wires. This factory recommendation must be adhered by to maintain the validity of your new car warranty and extended auto warranty, if applied.