I feed mine 1 cupful [8oz] each per day - half morning and half evening.
If there is food left over in the bowl when I come to add the evening ration, I reduce the amount. That rarely happens.
I don't think my son gives them a full cupful in the morning as they are always very hungry in the evening when he has fed them. When that happens, I give a cup and half each in the evening. The rations were increased when the females started laying -- one of the females in particular is very hungry now she is laying.
Many days the ducks also get mealworms or defrosted pea treats during the day.