If you can take a look at Storey's Guide to Raising Ducks, he calls it pretty well on the usual age that the different sizes of breeds lay. And with my runners, they started acting out mating behaviors about four weeks before the first egg was laid. So that was a clue for us. I started setting out free choice oyster shell when they started playing hop on top, as Holderread (Storey's Guide) says to give them extra calcium a month before they start to lay.