How long has she been sitting, and is she still laying? It may be that she has not got a complete clutch yet and when she decides she has enough, she will start sitting in earnest.
Once she does settle down, she should not really leave the nest for more than half an hour at a time, perhaps only once or twice a day to poop and eat and drink.
If she has just not been sitting properly (and is not laying any more eggs) then you could either move her and the nest into a broody box where she is contained with the nest and food and water nearby, or you could incubate the eggs yourself. Usually, and I say usually as there is always the exception, nature is better than artificial hatching. I hope your duck ends up being a good mum!