It depends. Not all hernia surgeries are alike: where was the hernia? Was mesh used for the repair? Was the operation performed via minimally invasive surgery? What is the health of the patient? What are you defining as 'healing': return to work? Pain-free? Sports? Because of all these factors, the answer could be "weeks-to-months". This question is best answered by the hernia surgeon who did the case.
Abdominal wall hernia. Hernias in the abdominal wall are holes in the muscle layer through which abdominal contents can protrude. These can only heal in infants and toddlers who have a hernia at their bellybutton. In essentially all other circumstances, they will not heal and can only be repaired with a surgical procedure.