Dawn would be the best time to hunt ducks, but don't leave to early as ducks can arrive all morning long....I stay until at least 10am
Hunting ducks in the evening is tricky for two reasons
1) Most ducks will fly after sundown (in my experience)
2) You need to make sure that you do not hunt in there roosting area.....if you do don't be surprised if you go back to find no ducks using the area
There are other factors that come into play just like in deer hunting as far as time of duck movement is concerned......Moon, weather etc.