Every person is different BUT I started free ranging at 2 1/2 weeks. At first I put a fence up around the coop ( about 40' x 40')for about 3 weeks so they would get use to being put to bed in the coop at night and so they would associate the coop as their safe place. After that the fence got opened up and they now free range over 2 acres. By 6 weeks old they were let loose from sun up to sun down.
After loosing too many to count and about all my first hatchery order ( they were 3 months old when I started letting them go. I wait till they are at least 4 or 5 months old and almost ready to start laying .
Till then they stay on the starter/grower and the custom feed I mix.
I take them out only on warm sunny days after they are three weeks. I made a 5-foot by 4 foot box that I put netting on the top and so I won't have a hawk problem. I take them in at night and then at 8 weeks they go to the big girl coop at night, and a separate run during the day until they are a little bigger, then they go out with the big girls!