How often should one eat fast food?

Hey, I've tried every diet I can think of. I'm tired of it and I want to just start making positive dietary changes and exercising more and using all the good tips I have been reading over the years. I know fast food is not good for you. But, come on....when a person deprives themselves of some of the things they enjoy, it just makes you want it all the more and eventually the diet fails. I want to cut down on fast food, but, question, how much is too much. Should I eat fast food once a week or once a month? I welcome your opinion. Thank you.