I have 40 chickens, most of them are large breeds like EE's, BO's, BR, etc. I also have 2 pekin ducks.
I buy 100 pounds of feed and it lasts 2 weeks at least, sometimes 3. They also get some kitchen scraps. They ate up all the grass in their big run, so they are low on free ranging goodies right now.
I fill up a litter box full of 18% protein mash for them every day with oyster shell mixed in, and it's usually nearly empty by night. They waste some by spilling it on the ground, but the younger ones (bottom of the pecking order) usually hang around and clean that up.
Omran if you don't live too far from Versailles you should try buying their mash! It's only $15 for 100 pounds and I've found that they poop less and smell better than the other prepackaged feeds I've tried. Their scratch is 9% protein, I give mine a cup full a day tossed out in the pen. You can also buy 50 pounds of oystershell to mix in the feed for only $8.50. Since I started feeding them like this, they lay SO many eggs. Most of them lay every day!