My experience personally is not until they look mature and about the same size as chickens. We had a duckling who was just short of being fully feathered and our chickens attacked her viciously, trying to pull out her feathers and we had to segregate until she was big enough that they were afraid to attack her. Keep in mind, our chickens are not what I would call docile, we have big personalities in our flock. You might have more docile birds, but generally from what I have heard and seen, if they see weakness they go after it.
Edit: On the other hand, we have ducklings now that are at least half my chickens size and who at least under supervision while free ranging, are bold enough to chase off any older birds whether chicken or duck who get too close.....