In a healthy diet, how much and how often would you treat yourself?

Never... Never... Never...

By treat yourself, I assume you mean eat sugar, caffeine, alcohol, excess fat/oils, convenience foods, or calorie rich meat and dairy. These substances tend to be addictive for many people. It's kind of like a cigarette smoker trying to quit and then rewarding himself/herself by smoking once in a while. It just doesn't help you break the addiction.

Psychologically, calling a calorie loaded food a "treat" sends yourself the wrong message. Why reward yourself with something that is harmful and calling it a good treat? These foods are seductive, but some pleasures should be avoided completely.