i thought you had to be trained to be a spin instructor?
anyway, i'm a spin fantatic - i go up to 4 times a week in winter time, (too nice to be inside in the summer), and the classes are 45mins - 1hr.
music is important, clear instructions, time to recover (but not enough time to fall asleep), and variety. don't single people out in the class and tell them to work harder etc - seen it done a few times in the classes i go to, and it usually ends with words being said. the one thing i really hate is when an instructor makes you do stuff like press ups on the bars, arms work, dance etc - fortunately there's only one in my gym who does this and i avoid her classes. the standing up for 8, sitting for 8, then switching to 4 is quite good.