28 days. And one duck hatching is going to be a very lonely duckling.
Is there time to put in two or three more eggs? Only do that if you just put the egg in today or yesterday--otherwise you're going to have a staggered hatch that will cause trouble for a first-timer. But ducks really need company.
Also, hatch rates will tend to be between 60-90%, so if you only have one egg, there is a fair chance it won't hatch at all even if you do everything right. If you have four or five eggs in there, your chances of success are much higher.