It's entirely dependent on the individuals as well as what experiences they've been through. If they have a reason to mistrust humans, it'll take longer.
Some ducks will never tolerate being pet or held, but will stop running away from you. Hopefully you have some social and friendly ducks in...
Consistency in your routine and patience. Find out what kind of foods they really like and say something fun and cute like "ducks ducks ducks" when coming out to see them. Try to converse with them, when they speak to you, say something back. Eventually they will realize you are trying to...