Once about every two months if you include delivery pizza. Once every 2 years if you’re talking McDonald’s or some such.
About once a month, not counting pizza. And the only place I’ll go is In’n’Out. If you count pizza (which I don’t ;)), at least twice a month.
I don’t consider Steak n’ Shake to be fast food, I dunno if the OP does or not. IMHO, any time you sit down and they bring you the food, and you eat with read utensils, not plastic, it’s not fast food.
Even simpler, it’s not fast food if you don’t have to unwrap it.
Burgers and the like: once every two or three months.
Delivery pizza: about once or twice a month.
Once every three weeks, tops, only if I didn’t bring lunch for some reason and our cafeteria has nothing useful to eat.
From the response I guess Dopers must be healthier than the average American.
Yeah, but I get it via drive-through, so it’s fast and I do have to unwrap it. 
Once a month with the kids on Dad’s book club night, although lately I’ve developed a Pick-up-Stix kung pao chicken habit, every other week or so. At least that has vegetables.
I also discovered these horribly addictive microwave sausage biscuit thingies at the grocery store. I am trying to ration myself to one box of 3 per week, but I’m PMSing, too.
If you mean McDonalds, Wendy’s type places maybe 4-6 times a month.
If you mean pizza, buritos, falafel, deli sandwich type places then just about every day.
Ahhh. Perhaps the OP needs to quantify what’s fast food, and what’s not.
It depends on what fast food really is. I don’t think I could even eat McDonalds or anything like that any more. Nor have I been to a McDonalds in years. Makes me sick to just smell the place.
On the other hand I would say I eat a pizza every other week or so and will goto Subway every few weeks. Other then that I can make my own food thank you very much.
Yes, I think we need a definition. I eat pizza about once a week or so with friends, and fries from the place across the street (very dangerous- never move into an apartment across from a chip shop) about once a week. I’m trying to resist, but I am a terrible french fry addict.
I think I last ate a hamburger about two months ago.
During the school year, 1-2 times a week. Almost exclusively at Chik-Fil-A, Blimpie and Mr Gatti’s. I meet one of my friends at the Union for lunch 2 or 3 times a week, and sometimes I don’t have the time/ingredients/desire to pack a sandwich. I also have lunch with my dad about once every week or so at Mr Gatti’s.
During breaks between semesters, once every week or two. Usually Subway, sometimes McDonald’s, Burger King or Raisin’ Cane’s (a local chicken finger place). We (my family) also order from Domino’s about once a month.
Once a week, every saturday morning, breakfast burritos from a local restaurant. MMmmmmmmm.
Not so much, otherwise. Maybe 3 times a month for burgers or tacos.
My job has me in a car driving around from store to store every day. So I eat fast food every day Monday - Friday.
If I’m feeling lazy/sick/or in a hurry, I’ll eat it on weekends too.
Oh, and I never super size, usually I eat the happy meal or just a burger and skip the fries. I’m short and could lose 10 pounds.
So the short answer, 7 days a week.
Maybe once a month I’ll get a slice of pizza and a salad at the mall. I wouldn’t put it in the ‘fast food’ category, but you might, so there you go.
And three or four times this year I got a deli-style sandwich from Arby’s. No fries, though. Arby’s fries suck.
That’s it.