If you set up for it you can have chicks living full time with the flock by roughly 4 weeks. I just moved my chicks into the coop tonight, exactly a month from the day I brought them home.
This is my brooding and integration article...
The hen needs to be broody before you introduce the fertile eggs, or they will not sit on them or accept hatched chicks.
There's never any guarantees with living animals. Even a properly broody hen can accidentally or deliberately kill the chicks after hatch, or other flock members could do...