ive heard three main things about the length of a dream. i was woundering which is true.
the first (learned it here): you have a 15 min rem cycle followed by a 15 min or so dream, another 15 min rem period followed by a 30 min or so dream period, etc.
the second (also learned it here): you start out with alpha beta and gamma phases of sleep (lasts about 2 hours) followed by the rem/dream thing.
the last ("learned" it from a teacher): dreams last a couple of seconds.