I have a pair of Mallards !?

A drake and a hen. The hen has laid about 12 eggs and is sitting on the nest in their pen. If they are viable fertilized eggs how long will they take to hatch ? its been about 3 weeks now. She is normally really skidish but now she comes at me in attack mode when I go in to feed the the chickens. I can only ume she is protecting her brood ! What do I need to do or not do for the ducklings ??