It depends on if they are free range or not. If they are, then they don't need any at all right now. If they are getting decent laying hen feed, you actually don't need to give them any scratch at all. Think of scratch as dog treats. Your dog does fine with kibble, and the treat is extra. I would not give them more than a cup or two a day at the most. Another thing to consider is that when you give hens (who are confined to a run) a bunch of scratch, they get a lot of energy and no place to burn the calories. This leads to meanness, like feather picking and fighting, simply b/c they are bored and have all this energy to use up.
I have 40 birds, most of them young pullets and roos, mostly banties, and they get about 4 lbs of feed a day, plus occasional kitchen scraps and garden junk, and maybe one ear of dry corn if it is cold out. This is their summer chow, and the rest of their intake is up to them to find out in the timber and yard. They do perfectly fine. Sometimes when I am out late they miss their evening feed, and they seem none the wiser.
Try checking out Gail Damerow's chicken raising book for her suggestions on actual feed amounts by pound.
