I eat out once or twice a month, sporadically.
Some months, I don't. Some, I do. My only rule is that it needs to be something that is either difficult to prepare at home, or unique enough that while I could prepare it, I don't want it in the quantities that economic home prep would require.
For example, I usually go to my favourite nearby gyro joint for my monthly splurge. I love gyros, but making them at home is not very economical for me, and most store bought variations are gross. Another is for desserts - I could buy a whole cake for the cost of a slice at a restaurant, but I don't -need- a whole cake.
I don't ever go out for sandwiches or burgers, and I'd never go out for pizza, homemade pizza is practically free if you buy the basics in bulk.
Sometimes, I crave things. And I figured out years ago that denying my food cravings makes my normal food (which I enjoy) seem less good. So, I indulge the unique cravings (gyros, slow roast bbq, korean bbq, etc) that are impractical for me to make for/by myself, and soldier through the rest. I have a $20 eating out budget that I simply use or don't use, as I see fit.
Regards,