That varies from person to person depending on how quickly they are able to metabolise (break down) the caffeine molecules. Different life and environmental factors can influence this also. The time it takes for the body to eliminate one half of a caffeine dose is normally between 3 to 12 hours. It is called the half-life of caffeine. Several factors can shorten or lengthen the half life of caffeine. Smoking, medications and diseases are some factors. Pregnant women have higher caffeine half-life, 18 to 20 hours – pretty much longer for the fetus since its organs are not yet fully formed to be able to eliminate caffeine easily. About 99% of the elimination process takes place in the liver.