Supposably the rule is that the THC is all absorbed after 5 seconds., I personally think it differs but probably just a small amount since: 1. All people have different lung compacitys and therefore have a different amount of surface area to absorb the THC w/. 2. I assume it depends on how big of a hit you take.
Now I don't know much about lungs and how they work, so I really don't know how THC is abosorb, and maybe niether of those factors matter.