Oh and with my 2 pekins I'm going to have to buy my second bag of food next week. This last one made it over 6 weeks. Pekins are known to eat a lot too :)
I use the grower/finisher Dumor from TSC. It's $13 for 50lbs, but I have 2 drakes. Since yours are ducks you would need layer rations. There are several different brands but chicken layer is what you are looking for. As for how much to feed them... My dogs eat the duck food I leave it out...