As soon as they start growing feathers. So, within hours of hatch. Even on a day old chick if you spread the wing and look at the tips you can see the flight feathers starting to come in. By 3 weeks much of the body will have a smattering of regular feathers and by 6-8 weeks they should be largely feathered out.
ISA Browns are hybrids so do not necessarily breed true. Usually their offspring are red-brown, but just like ISAs themselves vary you may get a variety of shades in your chicks. And if you bred the ISAs to a different breed of rooster the possibilities depend on his genetics as well.
