Dang it I just typed up an answer and then closed the window instead of clicking "Post"!
I think the difference between the mix of the idle and what ever other circuit you are testing will determine how long it takes for the idle to change the spark plug. You probably know that all the instructions for plug chops (dunno whence that term came) say to run the bike at the desired throttle setting for a while then simultaneously pull the clutch and hit the kill switch, so that the spark plugs won't be influenced by the idle mix.
On the other hand, my R65 used to have a right carb that somehow leaked fuel around the needle jet, making it run rich (8 stroking, sometimes) and blacken the plug. I was doing test rides for the more experienced people at a tech day, and the minute or so of low speed running and the idling on coming back to the tech day were not enough to clean up the plug from that richness. Assuming my idle mix is lean enough to do that, I don't know. But when they subbed in a much leaner than stock needle jet, the plug cleaned up fairly quickly (2 miles? I'm guessing.) and the low speed and idling I did on returning did not make it darken back up. So the effect of the idling isn't immediate, but for the clearest results don't do it if possible.
Was this a theoretical question?
