Cobwebs/black soot & oily plugs

[Lawson Cox]

I know I'm older than all y'all, but I've forgotten most of what I knew in my youth, so help me out. Apatchy mostly sits nowadays but does get cranked occasionally to keep everything lubed up, etc. The plugs used to be tan when I drove him a lot but now they're black. That's because it isn't run long and hard enough to get the dust and cobwebs out, right? or at least almost right. LOL

Thanks for the refresher course. How long and hard and how often does he need to be driven to be tan again?