How long should a broody sit before given chicks?

[losttexan]

Like many others here, upcoming Spring means firing up the 'bators and collecting eggs. I have an average of 70-100 eggs cooking at any one time.
One of my little 29-week old Silkie projects decided she would go broody (believe it or not, my Silkies don't seem to be as broody-prone as others). I moved her into a maternity ward and gave her some fakes to smoke her out. She does seems committed and has been sitting tight for about a week. I'm wanting to give her some eggs to hatch to evaluate her for setting and chick-raising skills (I do appreciate a good momma hen), but I don't see any need to put new eggs under her and have her sit the full three weeks (brooding is hard on them, and I don't like how much weight they drop). I will have some eggs going into lockdown today and the set behind them due March 10th.
My long-winded question: is there a consensus on how long a broody should be allowed to sit before shorting her brood with started, incubated eggs? Would giving them to her on day 18 be too short? Once she starts hearing those internal peeps, does it matter? Granted, all chicks are different, and everyone's experience is likely to be anecdotal, but I'm just seeing if I can identify a trend.

 

[ChickenCanoe]

IMHO, 15 days is likely sufficient but the longer the better. If it is warm where you are, you can put them under her at night and check back every 20 minutes or so for a while to make sure they're all still in the nest.
I put them under the hen while the hen is sitting tight in the dark. Then I don't mess with her but rather go into the building through another door and make sure no chicks are running around loose.
Good on you for not wanting her to sit a whole month.

 

[losttexan]

I've snuck live chicks in a couple of times and have generally been successful with it. What I'm wanting to do here is slip some eggs in when they're on their last few days to see how well she handles hatching duties (does she freak out at the sound and movement or does she sit tight and keep them warm and humid). I'm just trying to guess how long she'll need to sit on the real eggs to get good and bonded before the chaos of hatching starts.

 

[Shezadandy]

I've had my eggs hatch on day 19 under broody hens multiple times now. They're always fine with it. Did have a hen that cracked the first egg that pipped on her first hatch ever- I think she was startled and gave it a good whack with her beak- so I treated that egg as a bit of an assisted hatch using bacitracin to coat the lining since the breached shell indeed led to shrinkwrapping- she didn't repeat on any of the other 8 eggs and all 9 hatched healthy and happy. We named the whacked shell chick "Lucky". Don't think I'd shortcut them much more than day 18 or 19, though.