My experience of Terrence McKenna's speech at the 9/14/96 Austin,Texas Whole Life Expo
He mentioned that, in general hallucinagens tend to breakdown barriers; which (he claims) are of an artifical nature, and are set up by society out of greed and fear.
Though, in his book he mentions the time of 8 minutes for the high, in the experience he mentions in this speech he corrects this to 2.5 minutes and another similar amount of after effect.
Fortunately I was able to find out a little more about the experience beyond the "official" story. The grades of DMT out there do vary, as do the experiences. Not everybody actually sees the elf/machines he mentions. With less potent versions of DMT others seem to get a much much shorter version with the communicating elf only being sensed, but not actually seen. One person related it like this: "By the time I took a hit and handed it to the next guy I was back down." An obviously less potent batch, he also mentioned that there was the fear factor still involved; which anyone who's ever been on a bad Acid trip will attest to. The most interesting aspect of this person's account was the feeling of ETERNITY he experienced in this extremely brief instant. He said it felt like he had been in there forever; even though it had only been a few seconds. Apparently IT allows one to access reality outside of TIME.
Though he says it is the most powerful hallucinagen (not a proper term at all), he says it's also the safest; that it is naturally produced in the body.