One bag of feed last me about a month and a half and yes you will go through it faster in winter since. In the winter I put cut up greens and scratch grain in their water.
Ducks usually need a closed in coop at night to keep them safer but the coop doesn't need to be as complex as a chicken coop. Chickens need the run and closed house with nest boxes, but in my experience ducks only need an A-frame coop with a semi closed in area for nesting. My duck coop has half...