Mine "usually" start to lay a week or so after they wean their chicks. By weaning their chicks I mean they no longer sleep with them at night and the hen's spend their time with the rest of the adults during the day and leave the chicks on their own. Mine usually wean their chicks somewhere between 4 to 9 weeks of age, so there is no hard and fast time.
I had one that started laying 2-1/2 weeks after she hatched her chicks. She did not wean her chicks until they were 3 weeks old. Those chicks did not know what to do when the hen was on the nest laying. They just wandered around on their own until she came off. That's why I said "usually". They don't all always do the same thing.