At least a couple weeks. You want the other birds to forget her so when she re-enters the flock she will be like a new bird. Hopefully, she will be so busy protecting herself that she won't have the opportunity to pick at the other girls. If the behavior continues then she will keep needing to be locked up until she is re-educated. It's a tough habit to break, I won't lie. I tried everything last year when dealing with the same issue. I ended up having to cull the highest tier of my pecking order out to stop it. I never could get them to stop. I had one low girl that was able to be saved. She is still with the flock.