Well, I wouldn't even begin to know the answer to that, but the age of the chickens have to be a factor. Are you talking about adult chickens?
I am feeding somewhere in the neighborhood of 45 ranging in ages from 3 days to 2 weeks to 9 weeks and I *think* I go through a 50lb. bag a week. It's hard to determine, really. You have to take in to consideration the amount of food loss (scratched out of the feeders), the size of the chickens, whether they are actively laying or not, the season (summer/winter), whether they free range and how much 'food' they get on their own...
Maybe someone else can give a close estimate if you provide a few more details.