Depends!
 
Sorry but that is really the only answer.  
 
Depends on the quality of the free range land and what foods are available to the chickens.   For the most part the more varied the landscape the greater the supply of foods for the girls. 
 
Depends on how much "quality" you want out of your eggs and meat, are you willing to settle for 3 eggs/wk due to low nutrition or do you want 6 eggs/wk per chicken.
 
for today's chicken keepers, Free Range is mostly to supplement what we provide for them.  There was a time back in the day where the chickens were mostly on their  own in regards to finding food, but they probably had multiple manure sources to pick through for bugs and leftover seeds and the egg/meat production back then was probably less then today's levels.
 
Free Ranging will cut down on your use of commercial feed but how much you can't know in advance.  Only time will tell.