Personally I don't think I could ever do it, after they've gone to all the work of sitting and hatching them, it doesn't seem fair to just take them away. Why not let her raise them for a few weeks and then you can sell them as started pullets?
My first broody kept her chicks with her for 15 weeks! But my most recent broody gradually gave them more and more freedom from about 3 weeks on. Her chicks will be 5 weeks old tomorrow and the Mama laid her first egg since going broody just this afternoon. I noticed last night that her chicks were sleeping on a different roost in the coop than their Mama so I think she is pretty much done with the parenting gig.