A whole friggin lot. Since I'm living with my family...I'd say minimum 4 nights a week, plus lunch most weekends.
I don't cook for myself much, because if I buy groceries/ingredients, someone else will use them, and if I do cook, I get annoyed no one eats the leftovers with me and they go bad. There's no room in the freezer really, plus with the baby it's a PITA to just cook for me.
My family pretty much won't eat home cooked meals, and don't cook themselves, so it's way easier to just have them pick me something up. Except they eat fast food for almost every single meal. I don't know how they're not sick all the time. To them, Applebee's is "real food", like a quality meal.
I try to cook/eat home the nights they eat real crap (taco bell is a weekly dinner for them) and convince them to at least pick up sushi or greek takeout other nights.
My BF also eats tons of fast food. Which is weird, because he buys all organic groceries, takes 5 million supplements, juices, etc. If he's off, he'll usually cook, and if he gets home early, I cook for us, but that's never more than twice a week.
When I lived on my own I usually ate out or did takeout about twice a week, but usually social stuff. I imagine it'll be the same when I move out again, maybe slightly less