it is of course sometimes hard to know what is in a chickens mind, but my guess is your other d'uccle was freaking out not because she needed to lay an egg, but because she saw the nest being invaded by strange little mouse-like fuzzy creatures that needed to be removed. And how could she possibly know what they were, since she had never seen a tiny chick before?! And then the other hens saw the chick(s) as potentially tasty little morsels & began to play keep-away. Fyi my hens have hatched probably 3-4 HUNDRED clutches through the years by now, & the flock long ago learned that chicks are just miniature chickens. The flock members fully accept the chicks & treat them as just another chicken.  Mama & her chicks can take dustbaths with the flock & wander around the yard without the others bothering them in any way. The main reason i still give every broody a private space to live and raise her chicks is because all the broody pens are wrapped in 1/2" hardware cloth, (same as the main runs & coops). I know a snake or rat cannot get to the chicks in the broody pens. That COULD happen in the main coop, since my chickens free-range dawn to dusk every day, meaning the door to coops and runs are wide open.  Also, broody hens just seem more relaxed and content having their own private space. To answer your question "how long should separation occur?", i would treat your situation as if u had a pet dog that had just killed some chickens/chicks which it had never seen before. I.e. watch the other hen's behaviors VERY closely to observe their reactions towards the chicks. Are they still acting like the chicks are invaders in their space? A tasty morsel they need to get at? If/when the others begin to ignore the chicks, then u can allow the others to have access to mama & chicks, under your close supervision. Are the others still ignoring the chicks? Is mama hen quick to protect her chicks if the others so much as look her chick's way? A broody in full protective mode seems to suddenly grow 3-4 times her real size. She will fluff up every feather on her body starting with raised hackles, spread her wings wide and away from her body, lower her head to the ground, and be ready to charge and attack any perceived threat to her babies  Having said all of the above, i understand YOU were very stressed out by the "murder scene", & that u dont want to risk losing more chicks.  Based on what u described, plus my prior experiences, at this early stage i dont think u can possibly predict "how long separation should occur." At the least, allow the chicks to get strong and mobile enough to easily keep up with mama. Maybe in a week or so, after careful observation, u will decide the others likely wont hurt the chicks. At the worst, to be SURE the chicks arent harmed, they may need to stay separate from the others for several weeks. (Due to the typical docile nature of your breeds (d'uccles, silkies), i really dont think it will take nearly that long. My chick killers were isa browns, which are KNown to be intolerant to new chickens).  So flock acceptance may take place quickly (within a week.)  Whatever u decide, the main thing is to enjoy the experience. Watching a broody with her chicks is amazing.  Just as nature intended!