Hey Phil;
Hard to say. It would depend on the temperature inside and the type of metal things are made of and how many times things have been cleaned and plugged /n-plugged.
As we clean things we also remove the coating that is on the metal to help stop oxidation and provide good contact. As the coating is worn away from cleaning and repeated plugging / un-plugging, oxidation, with heat as a catalyst, happen quicker.
Just more thoughs.
Cheers
Bruce