8-12 hours roughly.
Rigor mortis is the stiffening of the muscles postmortem due to chemical changes in the myoplasm. It begins after death but is usually not readily detected until hours later. The stiffness progresses with time and becomes maximal between 8 to 12 hours postmortem at room temperature. Rigor develops in the resting position that the body is in at the time of death, and when fully developed, is strong enough to support the body by the head and ankles (Image 24.24). If the rigor is broken during its development by movement of the body, it may redevelop in the new position. If the rigor is broken after maximal development, it is unlikely to redevelop. Rigor mortis that is antigravitational or not consistent with the position of the body indicates that the body has been moved (Image 24.25). Factors that may hasten the onset of rigor mortis include infection, terminal seizure or hyperactivity, electrocution, increased body temperature from other causes, and increased environmental temperature. A cool environmental temperature will slow the onset of rigor. Bodies with decreased muscle mass (infants, the elderly,and the markedly obese) may not seem to develop full rigor because, by deﬁnition, rigor is due to muscle stiffening. Once rigor mortis is fully developed and decomposition continues in the muscles, the rigor will begin to pass off, again at a rate that is dependent on the same factors as those that affected the development of rigor. Differentiation between rigor mortis that is in the process of developing and rigor mortis that is passing or fading may be based on the presence of other postmortem changes that could be subtle at that point. Discoloration of the skin, including early marbling, slippage of the skin, early bloating of the face, and a distinctive odor may accompany the passing of rigor mortis.