In my opinion, and depending on how much room you have, when the chick is 2 days old is a great time. I'd definitely try it while mama is still protective of the chick. She'll take care of most of the integration issues.
Some people have reported disasters when letting a hen raise chicks with the flock. You are dealing with living creatures and anything can happen. It does depend on the personalities of the chickens involved and how much room you have, but what "normally" happens is that a chicken pecks a chick, mama severely reprimands the offender, a lesson is learned, and there are no more problems. When you let them out, monitor for a while and see what happens. Some hens will go after young chicks. Usually the rooster is protective but sometimes they can be a problem. And it depends somewhat on how protective mama really is. A good mama will take care of her chicks.
Good luck.