It takes 28 days just like any other kind of egg or just like chicken eggs.
It Taked Approx 28 Days for a duck to stay on her eggs , Conor Cunninghaam xP;
Mallard duck eggs take 28 days to hatch normally, plus one day for the older eggs in store, say 29 days.
On average, 28 days give or take a few. Some hatch sooner, some later, but after day 35, I would recommend throwing out any that haven't hatched to prevent any possible rotten ones from busting open...
More or less....21 days.