You should take them off the chick crumbles when they are about two to three weeks old. Gradually add the new feed - just a little more each day so it isn't a sudden change.
I wouldn't start them on layer pellets though until they are four months old. Look for a pellet that has about 16% protein to give them when they switch from the chick crumbles. We have a type of turkey finisher we use for that.
Also it is good to find a source of extra niacin for them, like brewers yeast in their feed or niacin tablets added to their water. Most feed is designed for what chickens need, ducks need more niacin, without it they may develop leg problems, especially Pekins.