By rendering the fat, you remove all the water and protein. If you are rendering suet, you are left with a very high saturated fat tallow. Saturated fat is pretty stable stuff. If you protect rendered suet from light and water, it should stay edible for years.
Rancidity occurs three basic ways: via oxidation of unsaturated fatty acids (you avoid this by having a high saturated fat mix to start out with), via reactions with water, and via microbial digestion. Pemmican avoids all of those through rendering the fat, thoroughly dehydrating the meat and hopefully being stored in a waterproof container.