Tetrahydrocannabinol interacts with CB1 and CB2 receptors that are spread out all over our brain and switches the mode of your endocannabinoid system the way you want it: you feel euphoric and impervious to pain. The highest thc strains the THC level in the strain, the more receptors are involved in the common work.
The potency of weed (plus some other factors, like the way of consuming, user’s experience, etc.) also influence the timing of the high effect. Usually, the high effect comes within a few seconds after the first puff and lasts for about an hour or two. But in fact, TCH compounds stay in the human body much longer, and for a frequent user, it can be even up to two months after the last usage.