If you're talking about smoking, the onset of action is almost immediate. The concentration in blood rises rapidly for about the first 10 to 15 minutes. Then it levels off while the concentration in the tissues continues to increase, because the solubility of THC in fatty materials means that it tends to leave the blood and accumulate in the tissues.
Similarly, or rather mirror-image, when it is leaving the body, it leaves the blood fairly rapidly by metabolism and then excretion, but it leaves the tissues slowly. Therefore, it continues to be leached out of the tissues into the blood, carried to the liver where it's metabolized, and then excreted by the kidneys rather slowly over a period of what can be up to three days or more.