I let my broody hens make those decisions. They know more about being chickens than I ever will. The unhatched chicks that have internally pipped talk to the hen through the eggshell so she knows they are on the way. Before they hatch the chicks absorb the yolk. They can live off of that nutrition for over 72 hours before they have to leave the nest. Nature did it that way so the early hatchers can wait on the later ones. When they get hungry or thirsty they tell Mama with their plaintive peeps. The broody generally knows when it is time to lead the chicks off of the nest.
I've had some hatches over within less than 16 hours of the first one hatching. I've had some stretch over 48 hours, those can be stressful. That can happen in an incubator or under a broody hen. I had one under a broody where the first chick hatched on a Monday night and the hen did not lead them off until Friday morning. All of the chicks were fine.
I watch what is going on but try to leave the broody hen alone. The more I interfere the more likely I am to do damage. Of course if I see a real need to interfere I do. I practically never do. I have food and water on the coop floor where the chicks can get to it when Mama brings them off. And I watch. That's about all I do. The rest is up to the hen.
