If you keep them contained where all they eat is what you feed them, a simple rule of thumb is to give them enough scraps that they can finish them up in ten to twenty minutes. That simple rule of thumb should keep you out of trouble. The general idea is that their regular feed provides a balanced diet and you don't want to upset that balance too much.
Many of us do not follow that rule of thumb.
If you free range your chickens, all bets are off. They are going to eat whatever they want to. I throw my scraps on a compost heap and they get what they want.
If you really work the nutrition angle and study it a lot more than I am going to, you can come up with your own balanced diet. That is advanced nutrition and beyond my scope.
Getting back to the rule of thumb, that balanced diet from their food is an efficient scientifically formulated balanced diet. That will keep them growing and developing at an efficient pace, assuming you want them grown out and laying or ready to eat as soon and cost-effectively as possible. They will get all the nutrients they need in the right amount for efficiency. If you feed them treats, they might be a little high or low on protein, or maybe fiber compared to the optimum. That does not mean they will immediately die or that they will not develop into good chickens. It means they may not grow as fast or maybe that they will grow a little too fast. To lay eggs safely and efficiently they need to grow and mature internally at a certain pace. If that balance on growth and maturity gets a little out of kilter in our small backyard flocks, it is not usually that big a deal, as long as it is not a lot out of balance. If you were a commercial operation of five hen houses, each with 5,000 laying hens, then a small percentage difference would make a real difference to you. But a backyard flock of 4 hens is not going to notice a 10% difference.
The guidelines are 10 to 20 minutes. The further you get from those guidelines the more risk you take, but as long as you don't get ridiculous, I don't think it is a huge risk.