I see, so feeding them once a day? Man, I was hoping to have chickens for free lol.
Having chickens for free is a pipe dream.
You will get out of your chickens what you put into them.
Depending on how much kitchen waste you have in relation to how many chickens you have, that can help but you can't feed them things we shouldn't eat - like too much bread, anything else with too much salt, sugar or lots of artificial ingredients or anything else humans are prone to eat that have little nutrition.
If you ever said where you live, I missed it. As others have said, that determines what portion of their intake can be supplemented by natural sources around them.
The breed you raise is also a factor. Some breeds are excellent foragers and some are virtually worthless at finding their food.
I free range all by birds but where I live, there's virtually nothing to find out there from December to March. They eat about half the feed in May than they do in January.
Regardless, I still keep feed available all waking hours. Chickens are voracious eaters. If no feed is available when their crops are empty, they'll eat something. That could be bedding, feces, tree bark, rocks, whatever. I'd rather they eat feed.
When optimal nutrition (provided by feed) is diminished, production and possibly health and longevity will be curtailed.
The nutrient requirements of chickens are well known.
http://extension.missouri.edu/p/G8352
If their food intake is deficient in some, they'll likely be compromised.