The window is about three days, for the hen and chicks...you can judge it by when she leaves the nest..you can still slip a chick or two under her at night but the chick can't be too old either. You can tell if she acepts it by if it snuggles in and she lets it. Depending on how needy the chick is too, I've had chicks from different broodies 'swap' chicks the same age. After a week they are certain who their mommy is and vis versa
Silkies are known for their broodiness, aren't they? All you can do is try it, and be ready to take the chick back, I believe chicks are better off with a hen if possible. Some hens are more receptive than others
I had a broody PR that constantly tried to sit on eggs in the nesting box. Sometimes she'd fake sleeping in the nesting box while hiding eggs underneath her wings. Usually, I was able to reach underneath her and scoop the eggs out. After which she would hop onto the roosting board.
Anyways, a friend of mine was raising a couple of three week old chicks and had to leave town for a week and a half. I took care of them in his absence and during a warm day, I let them out in my front yard to play. Meanwhile, little Miss Adventure was sitting on eggs again. Thinking of an idea, I scooped her out of the nest and let her in the front yard. It wasn't very long an she was out foraging with the chicks and acting like a mommy for them. It took some effort to heard her into the backyard with the other hens.