I had one that started laying the day after she left her chicks (at around 2&1/2 months old), one that lost her eggs twice, and started laying 2 weeks to the day after, and one that has started laying already, and still has chicks with her. They just all pile in the nest together. It just depends on the bird, but I would think pretty soon after they leave the chicks, just hard to tell how long that takes!
Our White Leghorn sat and raised a small clutch this summer. The chicks are now 9 weeks old, and Mama just started laying again this week. She still has her "babies" with her, and they patiently wait on the side of the nest box while Mama lays her egg, then they all go back out in the yard when she's done.