Likely 3 days, that's been the average for our hens. The main thing to watch for is they will begin to stand in the crate instead of assuming the broody pose. Once I've got a full day of standing, I try putting them back. If they return to the nest ... back to the breaker.So I just finished building a broody cage, since we didn't have a dog cage. I put them in and they didn't seem very happy with me, but they have plenty of water and a buffet of food so they should be fine. How long should I keep them in for?
Anyways- I had a hen reject her newly hatched chicks, and had raised them myself for about 3 weeks --- then another broody hatched her own eggs. I had everyone in the bathroom at that point, the older littles were in one area and the new mom and her little bitties were in another.
The momma hen started talking to the older chicks, so I waited for dark and added one- and she took it ... added the others and everyone did great. They all bonded and that was that. Now that's a sleepless night and an early morning, because I did not (nor would I ever) just close the door and walk away, but by breakfast time it was clear everything was fine, the chicks were happy to have a mom, and the mom was happy for more chicks - and it did work out despite the age difference.
So, if you were still thinking of trying the hen that's been broody for 3 weeks, you could contain her in a spot where she can hear the chicks and the chicks can hear her - they may ignore each other completely- but if she talks to them, there's a chance she might raise them. I'm not saying it will work, and it will take dedicated supervision. Always add chicks in the dark. Most of the time when the hen shuffles around to make sure everyone's covered, and talks to them, things go well.