Hi. Spark plugs are changed when the end of their useful life is determined. The cleaning will depend on the condition that they present, generally in a very short time they will come back in the same way depending on several factors, for example, poor combustion, general state of the engine. Attached. https://www.ngk.de/es/tecnologia-en-detalle/bujias-de-encendido/diagnostico/fotos-de-bujias-danadas/ Salu2.