I would think right after they puff up and dry off. But only if she is broody, if not you must keep the ducklings in a brooder.
I don't understand... did you hatch them in the house? Or did she hatch them? Why do you have them if she hatched them? If you hatched them out in an incubator, she won't accept them back unless she happens to be broody and happens to already have been sitting on eggs for a while... and then she will abandon the eggs she is on, so... If, however, you have a broody hen who is sitting on eggs and has been for a while, and you already know those eggs are no good, then you can try to slip the babies under her at night and hope she accepts them. This is best done when they are super young--from a few hours to a few days old. Any older and they will be too large for this to work. If you hatched them in the house and do not have a broody hen, then you will have to raise them yourself. The female will not recognize them as her own, and most likely the adults will attack and possibly kill the babies. Good luck!
