The thing with the Mallard-derived breeds is that it varies a lot, much more so than would seem logical. Most sources will say that Mallards and the bantams breeds take around 26+ days and that the large breeds take around 28+ days. It is really kind of shocking that they are all the same species and can vary so much. The length of incubation probably depends on a number of factors. At our farm though, our bantams (Aussie Spots, Appleyards, etc..) usually hatch at 26, sometimes 27 days. Our Mallards usually hatch at 27 days. Most of our large ducks hatch at 28-29 days. Our Indian Runners usually hatch at 30 days. Confused yet? Of course, YMMV. I think, for whatever reason, duck eggs must be more sensitive to things like differences in temperature, humidity, elevation, etc... I don't know how else to explain the huge variations in length of incubation. My advice in order to find what works for you would be to monitor the eggs closely throughout incubation. Make sure they are losing the appropriate amount of moisture. Once the eggs internally pip, stop turning and move them to your hatcher. I think this is a better guide than moving them to "lock down" on any arbitrary day.