Well, that is a loaded question of sorts. If you are asking about wild or migratory ducks generally they lay in the spring and rear their young into the beginning of the summer. Sometimes they can lay two nests a year which depends on breed. Domesticated ducks are a whole different cup of tea. These are the ducks most people keep as pets and most in general do not fly. These will lay year round after they are 15 weeks old. They tend to lay about 200-350 eggs per year. You will have to research the different breeds and find out specifics for those breeds.
After mating, around july, basically in summer :D
Spring or summer
Ducks start laying their eggs around august. Their mating season is july.