I think Stephen King's comments are helpful in this regard. He says to write what you know. If you work or live around people who swear, quite frankly, you're probably going to have a hard time keeping it out of your books. In which case, I'm not sure you should even try. People swear - that's life. That's just the way things are. If a person doesn't want to read that, then that's entirely up to them.
And there are certain scenarios in which it's almost a must. If you're writing a war story and a guy gets his leg blow off, then he better darn well not be lying there yelling, "Oh sugar, it hurts." If an old grandma stubs her toe on the other hand, then she might say just that.
I work in a factory, and if I was going to depict life in the factory, I couldn't do it without throwing in some foul language. In all honesty, it actually makes me a bit uncomfortable to hear how some of the guys talk sometimes - but my views of decency and propriety don't change the way the world actually is.
If I want to avoid that, then I probably just won't write those kinds of stories.
I'd also mention briefly that if, on the other hand, you don't swear, you're not ever around people who swear, and it's very awkward for you to be around or write about those sorts of things - well, then it's probably going to come across in your stories as awkward and unnatural. Just like if you try to avoid using foul language when it seems very appropriate for you - if a character is screaming something bad in your mind, but you purposely don't put it down on paper, well, I think that tends to show. But if your characters wouldn't swear and you force them to just to try and make them seem more real, then it will seem unnatural, too.
I don't think there's an answer of "too much" or "not enough" to this question. It depends on the story, the context, the characters, and even what comes most naturally to you as a person.
PS: There are a lot of popular authors who have written stories with quite a bit of profanity. Stephen King, as mentioned before, being a prime example. It fits his books. There are definitely people who have a problem with it. He gets hate mail sometimes over it. Honestly, I think that's just ridiculous. If the reader doesn't want to read the F-word, then he just shouldn't read King's stories. The point being: don't try to write popular books. Write what you know.