I was going to say 28 days. I've only had the pleasure of hatching ducks twice, once when my hen set on some and hatched out three. To my surprise she tried to take care of them like she would her own chicks.
With other species of birds I've had them hatch a day or two early when there was a heat wave, and a day or two late when there was a cold snap. I don't see why ducks wold be any different.
Do you have a pan of water so the hen can bath? Very important when the hen is nearing the end of incubation. Duck eggs require a higher humidity then chickens. My Muscovy hen was setting fertile chicken eggs. I thought for sure they were going to hatch. Then she took a bath.

A natural act for a duck spelled doom to the chicken eggs. I think she killed them with kindness.