OP - ever burnt your tongue with a hot beverage? Now , imagine that pain/discomfort magnified by a power of ten.
Since the burn would be on your tongue, it would PROBABLY heal quickly - think a few days - a week tops, and every time you swallowed, chewed, spoke or moved your tongue it would hurt.
Cigarette ashes probably wouldn't do you much harm, depending on how many people you are talking about, butts might be a different matter since they are supposed to filter all the icky stuff out of cigarettes. Since they can make dogs sick, i'm guessing they would do the same to a human.
All-in-all kind of a dumb fetish to me, but if you're up for it, go for it. Probably wouldn't do much harm - probably.