Profanity in a young adult novel: How much is too much?

I am a first-time novelist writing a novel kind of geared towards older teens, but that I'm sure adults would enjoy too... my point is it's not really for 12-13 year olds. The main character, who is the first-person narrator of the story, swears a lot and I am wondering if it is innapropriate. She is 16 years old, has ADHD and probably a learning disabillity as well, gets frustrated easily and is kind of inarticulate (although she's actually pretty intelligent). She's also a little rebellious. The other characters usually don't use foul language more than once in a while. So I'm wondering, is this OK?