damn that sucks. Well how long it takes depends on how much melted onto your exhaust... I'd try to like scrape it off or get some sort of cleaner. Depending on the plastic you can use acetone (nail polish remover) or alcohol. I'm not certain which is good for your case especially since it's been melted. Also I haven't tried those on metal so you might want to look into that. Alternatively you could just leave your car running for awhile and just burn it off.