It really is better to switch the young chickens over to a 20% Grower/Developer/Broiler feed after the 1st 4-6 weeks on Starter feed.
It also depends if you choose to use a medicated starter (which I never would), but for those who do you do not want to keep them on that after 4-6 weeks. Switch to grower
You want to let them eat all they want but usually depending on the feed quality, 6 chicks will eat 20 # in 2 weeks but it depends on food quality and feed type and spillage. Be sure to give chick grit if not in the feed.
From a couple hatcheries:
A question we commonly get is how long to feed baby chicks "starter feed" for before switching to a feed called "grower" or "chick grower". The answer is: it all depends! Each manufacturer formulates their feed differently, so read the label and follow their instructions. Some only recommend the starter for 4 weeks before moving onto grower;
Baby chicks need to eat chick starter mash or crumbles, a blend specially formulated for their growth and development. Layer mash, crumbles, or pellets should never be fed to chicks, not even as an emergency ration. It has a high calcium content that is toxic to chicks and will cause bone, liver, and kidney problems or cause death. A good emergency ration is a 50-50 blend of rolled oats and cornmeal, whirled in a food blender to a mash or crumble consistency. They should not be on this emergency ration for more than a day or so, as it does not constitute a balanced diet. Starter feeds should contain no greater than 21% protein for dual-purpose or egg-type chickens – too much protein causes growth problems.