I've had my bike almost a year now (99 kdx200) and in that time I've changed the plug once. It was a case of too many cooks spoil the broth - my husband didn't realise I'd put a rag in the carb, I didn't realise he hadn't seen it, end result we left it in. I always carry a spare plug just in case, but have given them to people along the way rather than using them myself.
I'm a slow trail rider, but I've done just about all the mods possible - only porting & carb to go if I ever need it.
I also often follow an LT80 & PW80 around, so really slow going. I just make sure if I've been riding with the girls, I open it up on my own afterwards.