Question about when to give ducklings away?

We hope to hatch some call duck eggs (our ducks have gone broody and started sitting on their eggs) but if they hatch we will have to find new homes for the ducklings. How old should the ducklings be before we give them to their new homes? We can only provide for the three call ducks we have and we hatched all three ourselves with incubators. The ducklings we may have in the summer will be raised by their mothers. How long should we wait?