Plantation owners had two designated meal times a day. Breakfast was an hour in the mornings, like 10 am, and they had another meal slot in the afternoons, like 2-3. The amount of food they got was usually pretty sparse. Soups, gruels. Sometimes the food was literally put in a trough, like feeding barn animals.

Obviously, each individual slave owners "generosity" would be a case by case thing but typically every month, a slave would get an allowance of some cornmeal and some kind of meat like salted pork or fish. Frederick Douglass described this and that he got a bushel of cornmeal and eight pounds of meat a month. Sometimes slaves would be allowed to have and work their own gardens, called truck patches.

You may find it beneficial to read Narrative of the Life of Frederick Douglass for more information on this and other details of being a slave.