I'm not 100% sure, but I think your lungs do absorb more THC when you hold it in, but like somebody else already said that more tar also stays in there. I only hold hits in 4-5 seconds for bongs and i smoke blunts and joints like cigs.
Most is on contact. You know what a parabola looks like right. you will get significantly higher when coughing on the exhale and after 2 seconds is a waste
Ha. I remember as a teenager, trying to make every last scrap of weed last as long as possible, I would hold my hits in till I was almost blue in the face. I was probably getting high with the weed and killing brain cells from lack of oxygen.
Nowadays, I tend to take the first hit of a joint harder and longer than the rest. Probably 5 - 7 sec first few hits and 3 - 5 rest of the joint. Sometimes when I am on the computer I will just let it hang out of my mouth like a cigar and just keep puffing along almost forgetting I am toking and next thing I know I have a roach in my lips.
Imagine Michael Phelps' lung capacity. He could probably hold a toke for 2 minutes until there is no smoke.