That sounds like they are going through way too much feed! When I had 14 chickens, they would go through one 50 pound bag every five or six weeks. I am down to nine Chickens now and a 50 pound bag lasts about two months. Sounds like something may be eating the food besides your chickens or they may be spilling a lot of it. Try to make sure none of their feed gets spilled on the ground and wasted because I would think 20 hens would go through a 50 pound sack in three or four weeks. Mine don’t get hardly any chicken scraps, but I do give them about a cup of dried BSL every morning. They do get to forage every day in my yard for 3-4 hours.We do give them all the kitchen scraps, which is a pail-ful daily, and they free range half the day, but at about 20 hens they eat a large bag of feed every week. Are we overfeeding them? I wonder.