I would think the first one should hatch from being laid if she has sat on them the whole time. Hens lay a clutch but necessarily sit on them until they've completed the clutch, so the due date is 21 days from then. however, if your duck has consistently sat from the first egg being laid then I would think it would be 28 days from first egg being laid, and the others owuld follow as and when their due day came... however I could be wrong, and would be interested to find out about this because a hen sitting on ducks eggs (first natural hatch for both the hen and myself) so I am learning as i go. I kept the duck eggs as they were laid then put them under the hen altogether so they should all have the same date.
Keep us posted jayjay, this hatching business is exciting... if not a little nerve racking. I'm dreaming and tossing and turning thinking about them all night!