Estimates I've seen are to limit treats (veggies scraps, grains, scratch, etc.) to no more than 10-15% of their daily diet, for ideal health and egg production.
A very rough estimate is if you have 6 chickens who will each eat roughly 1/4lbs feed per day (1/4lbs roughly equals 3/4c), then you're feeding 1.5lbs per day total (4.5c). Out of those 4.5c, no more than 1/2 to 2/3 of a cup should be low protein treats. Take this all with a grain of salt, as your chickens may be eating more or less than that, but it will give you some perspective. Such as if you're feeding 5 cups of veggie scraps to them everyday, then you are way overdoing it.
However, if your treats include lots (I'm guessing here, but perhaps 20-30% of the treats) of meat, fish or insect protein (ie, mealworms), that will help better keep the nutritional balance in order. Animal proteins have much more complete amino acid profiles whereas grains/seeds/nuts/veggies have varied amounts.