im not sure about holding it in, from what i understand once the fire touches the green, the THC is vaporized and that only takes a few seconds to be absorbed by your lungs.
That's why when you give people a shotgun, the smoke has to come from your mouth, and you can't inhale it and give them the leftovers cus they will be getting smoke with no THC.
Honestly, I've noticed a difference in length over the years, but only up to 3-6 seconds. In other words, if you hold it past 3-6 seconds you won't get higher, but holding it 3-5 seconds gets you higher then when you let if out all the way...
more then anything, i've noticed it's not holding it in, but when you get all the smoke in your lungs, keep inhaling fresh air, or toking with the carb open, so that your lungs are filled to the tippy top..hold that for 5 seconds, and release slowly..if you feel like coughing, go with the flow.
Because one thing I do want to mention is that I have observed truth in the saying "you have to cough to get off" I've taken monster tokes that have caught me off guard and made me cough for 5-10 minutes straight and after one hit been pretty toasted..when normally my tolerance would have held up and even an entire bowl wouldn't get me that high.
Again when you cough you don't get oxygen...it's like holding it in I guess. We need some expert opinion i think.