OOC our language is pretty filthy, but it's always a sort of flippant, offhanded, non-angry cursing.
IC it depends on the personality of the character and the situation. When it's appropriate it's appropriate and they'll cuss as much or as little as needed. We'll have stuff as innocent as a cleric talking to and comforting a young child, and then the same session have the cleric cursing with righteous fury; we'll have innocent and lighthearted banter, formal language in polite high society, and then a half session later we might get sexual innuendo from a fullblown fiend. Whatever the characters and situation require is what happens, and we'll try to stay as true IC as possible.