If the ammount of corn being fed is either a) making them overweight or b) causing them to not eat enough of their nutritionally balanced feed, then it is too much cracked corn. Doing rescue over the years, I've come across a number of chickens who were fed solely or predominantly corn, and they were not healthy birds - especially egg laying hens. It just isn't enough calcium for them to remain healthy and still lay. It sounds like your birds are just getting it as a treat, but every night does seem a tad excessive. If you want to give them a treat in the evening, perhaps vary things a bit - corn one night, rolled oats another night, grass clippings another night, etc.