If it were up to me alone, my answer would be "never". I hate fast food because I know how unhealthy it is. I never bring home ANY processed (fast) foods from the supermarket because I know if it gets into the house, eventually it will be eaten.
I haven't had most fast food items for many years and I don't miss them. I found that if you stop eating them altogether, eventually you don't crave them.
However, I have a friend who likes eating out. So I join her to eat out about once or twice a month. And although we try to keep it healthy, it's often not up to my standards.
The best I can do is to suggest a full service restaurant or a buffet restaurant. At the Golden Corral, for example, I can find lots of healthy choices.