The answer is: the slave owners were able to legally work a slave only 21 hours a day.
The laws that regulated the life of the slaves in Louisiana, were stated in the Black Code of Louisiana (1806) and in the the Law of Slavery in the State of Louisiana, (1847). Therefore, the conditions of slaves were different from the rest of the country. These laws also stated other working conditions like free enjoyment on Sundays and a half hour for breakfast.