If by "black snake" you mean Black Rat Snake, they can lay up to 30 eggs, though most clutches are smaller than that. It usually takes around 72 days for the eggs to hatch, but unlike bird eggs which have to be incubated at a specific temperature and hatch after a certain time, reptile eggs can vary in hatching time, depending on the temperature and other conditions in which they are being incubated. The sex of the babies is also determined by the temperature.