Something kinda important everyone should know who indulges in the magical plant known as cannabis. The length of time one holds a hit of pot smoke in the lungs has little to no effect on how high ya get.
What is important, however, as most pot smokers tend to learn instinctually, is how DEEP one takes the smoke into the lungs. The reason for this is basically pretty simple: the lungs are essentially made up of a bunch of tubes, called bronchi. These tubes break into smaller tubes, that break into smaller tubes, that break into even smaller tubes, that finally end in tiny sacks called alveoli. These tiny sacks are covered in very fine blood vessels, called capillaries. In normal breathing, oxygen goes from the alveoli into the capillaries, which drain into the pulmonary veins, which go directly to the heart. (This is way drugs take effect so quickly when they are smoked).
So basically, to get your money's worth, the idea is to fill as many alveoli as possibly with smoke by taking the smoke as deep into the lungs as one can. However, the actual transaction of cannabinoids (the pharmacologically active chemicals in pot) going from the alveoli to the capillaries happens very quickly. All that happens when you hold the smoke in for a long time is that it gives the irritants in the smoke a longer time to irritate your lungs. It's pretty well established that regular pot-smoking increases one's risk of chronic bronchitis, emphysema, and other lung illnesses (although apparently NOT cancer, as several major studies have shown, which I'll discuss in a future article). So, to decrease your chance of developing any of these illnesses (chronic bronchitis, while nowhere near as bad as lung cancer, still sucks ass... let me tell you) you want to get the irritants out of your lungs as soon as the cannabinoids have been absorbed into the bloodstream. And this happens in a matter of a couple seconds.
Two studies (possibly more, but two that I found today, anyway) demonstrate this phenomenon. One study  looked at the cognitive effects of cannabis smoking in participants who held the smoke for a short vs. a long period of time. This study measured a battery of cognitive effects of pot smoking between the two groups, including tests of learning, associative processes, abstraction, vocabulary, and psychomotor performance. The majority of the tests showed no difference between the short-time and long-time smoke holders. A few of the tests showed minor differences, but the researchers concluded that this was simply a result of the participants holding their breath for a long time (maybe due to lack of oxygen). They concluded this because when the participants held their breath for a long time, they did worse on these tests whether they were smoking pot or whether they were smoking a placebo (a substance that looked and smelled like pot, but had no psychological or physical effects).
A similar, but slightly more complex study  looked at differences not only among groups who held the smoke different lengths of time, but also among groups who took in different amounts of smoke. As you might expect, the more smoke the participants took into their lungs, the more of an effect the smoke had. The more smoke they took in, the more THC was found in the blood, as well as the more the participants reported feeling high. Taking in more smoke also increased carbon monoxide boost, or how much carbon monoxide came out in the participants' breath. Basically, this is a measure of how much smoke the participants' lungs were exposed to. However, the length of time the participants held the smoke had no effect on carbon monoxide boost, nor did it have any effect on their self-reported high. The length of time the participants held the smoke also had no effect on an assortment of cognitive and motor tasks, similar to the first study. There was a difference in blood THC levels between the groups who held it in for 10 or 20 seconds vs. the group who held it in for 0 seconds, but that's kind of a no-brainer really. Frankly I'm not quite sure what the point of having them hold it in for no time at all was. But scientists can be wacky like that. At any rate, the article concluded that the study "...cast doubt on the common belief that prolonged breathholding of marijuana smoke enhances classical subjective effects..."
So there ya go. Breath it in deep, but the only thing you'll likely get from holding it a long time is a bad cough.
 Block RI, Farinpour R, Braverman K. (1992). Acute effects of marijuana on cognition: relationships to chronic effects and smoking techniques. Pharmacology, Biochemistry, and Behaviour, 43(3), 907-917.
 Azorlosa JL, Greenwald MK, Stitzer ML. (1995). Marijuana smoking: effects of varying puff volume and breathhold duration. Journal of Pharmacology and Experimental Therapeutics, 272(2), 560-569