here
http://www.drug-test-facts.com/drug-detection-times/
A study done by a group of doctors in Canada at Dalhousie University,
concluded that more than 90% of the THC is absorbed into the lungs
within the first seconds of inhalation. Another study confirms this,
but they put their number at 95% absorption. The 5-15% of THC that
isn't absorbed is due to THC loss in trace smoke, or smoke that never
enters the lungs.
here
https://cannabis.community.forums.ozstoners.com/index.php?showtopic=413
INHALING - Conventional wisdom is that holding in the smoke increases
the effects felt. Research shows the opposite; it causes more harm to
the lungs without increasing the amount of THC absorbed. Studies
indicate that 95% of the THC in cannabis is absorbed in the first few
seconds of inhaling. Holding in the smoke longer just allows more tar
and other noxious chemicals to be absorbed. Take small, shallow puffs
rather than deep inhalations.
and here
http://www.fast-times.co.nz/drugs/weed/index.html
Most people believe that holding smoke down for longer makes you more
stoned. BZZT - wrong! THC, the active ingredient in weed is absorbed by
the lungs instantly (no bullshit). That means all these years people have
been coughing and gagging and getting laughed at by their mates for no
good reason. So remember, holding down smoke does NOT give you extra stonage,
just more tar and other toxic crap.
not all the most reliable resources, but still, shows my point.