Ducks will start to lay between 18 and 22 weeks of age. Once they are laying though, they may not always produce eggs like clockwork. The reasons could be:
When your ducks moult (see the chicken health section) their egg production will drop and most likely stop.
If your birds have had a fright this can result in no eggs for a while.
Egg production is the first thing affected by dehydration. Make sure your birds have plentiful clean water.
If you are letting your ducks roam about in the garden they may have made a nest under a bush or in a corner somewhere. Follow your duck discreetly to find the nest - sneaky but necessary!