Ducks generally lay their eggs at early morning, around sunrise.
However, it varies a lot. Some ducks lay around 4:30 AM. Most lay around 6 AM. Others lay around 9 AM. And occasionally, a duck will lay in the afternoon, or even evening. Once I witnessed one of my ducks walking around at 5 PM when an egg dropped out of her. It never happened again.
If you want your ducks to lay in their coop, it’s a good idea to keep them inside until they’re done laying. Otherwise, they may lay anywhere in their yard. If you find that your ducks lay approximately at 6 AM, let them out at 6:30.
Unfortunately, however, ducks are perfectly capable of holding their eggs in until they feel like laying them. If a duck has picked out a special little spot under some wood pile 300 feet from their coop, she’ll lay there even if you let her out at noon.
In general, ducks lay during early morning, and they’ll probably have already laid by the time you let them out of their pen or coop. But if they scout out nesting areas beforehand, they’ll hold their egg until they’re given access to their chosen spot.