Rigor mortis is the stiffening of muscles after death.
Here's what happens. In both living and dead muscle calcium leaks through the walls of the muscle fibers, and once inside, causes the muscle to contract. Living muscle can transport calcium back outside the fiber walls, and thus relax itself. Pumping calcium out takes cellular energy (ATP). Dead muscle can pump calcium out until it runs out of energy. Once the dead muscle runs out of energy reserves, it cannot pump calcium back out. Therefore it stays contracted. This is called rigor mortis. Over time the dead muscle contracts more and more until it becomes quite tight.
Eventually, the dead muscle starts to decompose: the fibers start to fall apart, and the muscle gradually becomes pliable (at this stage, rigor is said to be "resolved."). This whole process happens only once -- there's no second rigor mortis, as the muscles have started to decompose and can't contract any more.
The timing of rigor mortis is quite variable and depends on the context (e.g. temperature) and the condition of the animal (e.g. its metabolic condition, what it died of). For example, rigor mortis sets in sooner when it's warm (37º C) than when it's cold (25º C).
In any case, here are some ballpark numbers. Krompecher (1981) examined rigor mortis in rats at different temperatures: