How often should I get duck eggs?

[1st time]

I am new at this. I have 4 khaki campbell ducks 1 male 3 female. They share a coop with 7 chickens. I have found 1 duck egg about 9/14 and have not found any since. Is there a time of day they will lay. I was told ducks are kinda funny. If you take there eggs they will find another place to lay and should always leave 1 egg in their nest. Let them all out of their coop aprox 6am monday thru friday and they stay in the run til about 2:30pm then they roam free on couple acres. On sat & sunday its aprox 7 to 7:30 am and are let out to roam about noon. They have a lot of wooded area to roam and was wondering if they would lay in the late afternoon or early evening. They were hatched 5/2/15

 

[1st time]

 

[1st time]

Do all of these white eggs look like duck? I got 1 real big one aprox 9/14 and all of the others 1 or 2 a day since. The brown are from my chickens