take a deep breath and see how long you can hold your breath for
now take a deep breath, hold it for 3 seconds, then exhale it entirely and then hold your breath
now im not really trying to push one side of the argument  but you can hold your breath for much longer when you actually have air in your lungs, and the only reason for this that would make sense is that your lungs are still absorbing oxygen after 3 seconds.
now i don't know the rate of absorption of THC molecules compared to the rate of absorption of o2 molecules in the lungs but i seriously doubt it would be any faster considering how much larger of a molecule it is.
i think that for smaller, less dense hits of smoke 3 seconds would be a fine amount of time, especially if your inhale is slow and drawn out
but if take a hit from something like a bong i seriously doubt its all been absorbed by three seconds due to the concentration of the smoke and the quickness of the hit.
what i do that i like to think is efficient is i take really drawn out, less dense hits so theres plenty of time and space for diffusion
yeah thats my contribution to ns for today