The answer is 8-12 hours.
Rigor mortis is caused by chemical reactions in muscles after death. Usually, parts of the body become stiffened because ATP is lost after death. After death oxygen concentration in the body decreases because ATP is still being produced and respiration is stopped. When there is no oxygen, anaerobic glycolysis produces ATP using glycogen. When glycogen is depleted, ATP production stops, and rigor mortis begins 2 hours after death. ATP is required during muscle respiration, but now there is a limited concentration of ATP, and its production is stopped. So, muscles could not be relaxed. 8-12 hours after death, muscle fibers are decomposed by enzymes or bacteria, and body relaxes, exiting rigor mortis.