In my personal life, I never use profanity, whatsoever. I just think it is unnecessary, however when writing it's a different story. In my first novel, my protagonist is a doctorate of English, Political Sciences, Journalism and a MBA. Yet in some scenes I find myself writing her saying the 'F' word. Not often, but enough.

In another story I am working on, I have character who are really bad and in bad situations, so the word is almost used in every sentance. I think I can justify that because it adds to the realality of the story, I guess.

I want to know, how often if any do you use that word.

I use the "f bomb" every time my character feels the need to say it.  I let the characters decide what they say.  There are several characters I have who use it frequently, though there are others who don't use swear words at all.  Some people are inclined to drop a swear in the event of an injury while others are are more inclined to say "hey, how you effing doing?"   Most people who don't use swear words will have reasons, ie religious background, parent who swears a lot etc.  Find out what the reson is if someone swears or doesn't swear.  If you know that character 1: Jane does not swear at all and then she swears, you know whatever it is that caused her to was serious enough to warrant it.  Character 2: Tom doesn't swear often because he has a family, but in his past he swore a great deal when he was a pot dealer; when Tom swears, you know he is aware of his surroundings as he's used to keeping an eye out to see if there are children, you also may have a situation where he is reverted to his previous drug-dealing persona ie, he's been moved back to the old neighborhood.  You also may get a great deal of swearing from Tom in the event of a major problem such as kidnapping his children, war, or prison.  Character 3:  Ashley was raised by her brother who is 9 years older than she is.  Her parents were killed in a car wreck and her brother had been given custody of her.  Her brother never had an issue with swear words.  He swore at times and partied frequently -- many of those parties being held at the house that he and Ashley shared.  Ashley learned, young, that she was popular with her brother's friends when she swore and nnow, she swears almost without stopping.  Each character should have their own speech pattern developed over time.  Knowing your characters's pasts will help you to figure out how they speak.  This may be your best bet to decide when your characters swear.

If you think this was helpful, but not helpful enough, feel free to personal message me and I will hep you further in any way I can.