Usually somewhere around R, sometimes X. I'm not shy about anything from drug use to prostitution (there's a Tanar'ri brothel right next door to the building in Sigil my players' PCs own) and the campaign has been exceedingly dark at times. I don't go out of my way to make it extreme, violent, or crude, but the themes of the game lend themselves to a mature group of players and a rather complex and mature plot.
I've had to edit out material more than once in my storyhour of the campaign, and there's an entire episode of the campaign that I cannot, under any circumstances, actually post here on enworld as part of the storyhour. I'll write it sure, but it's completely beyond the pale for anything that's proper to post around here.
My players range from 21-25, with me the oldest at 25, and so there's the tendency to curse like sailors at times, talk frank about certain topics etc. However we don't go out of our way to discuss such things, it just happens as the game and metaplot of the game progresses. My storyhour's first several sessions give a pretty good look at the overall tone and such of it all.