The THC is abosrbed almost instantly, and a normal breath is about 3 seconds, so by the time you breathe out naturally it'll all be absorbed. Holding it in allowes the tar in the smoke to stick to the lining on your lung, making you cough, and making it harder for the THC to be absorbed.

FYI a well rolled joint has the highest THC transfer rate as far as i know, so smoke joints.