My Araucanas will let the chicks go at a nice 2 months, sometimes 3 months old. . . My hatchery New Hampshire let hers go at 3 weeks old. I had an Easter Egger once let go of her chicks at, umm, 2 days old.
So, a very nice Araucana took over as a "foster mom."
Somewhere between 6 and 8 weeks, usually. She'll start to perch with them a week or two before, and sometimes she'll start laying a week or so before she gives them the cold shoulder. She'll start to reject them getting under her wings on the roost, and that's really when she's done.