The point of the rule of twenty is to keep things dynamic. The "Rule of 20" is loosely based on the notion coming out of my experiences in youth work. When talking to youth, the good rule of thumb is "age+3", so a teacher teaching a 7 year old should keep it to ten minutes. My assertion is that adult mental age bends this curve to about 15-20 minutes overall. Some adults can easily go longer, some get bored and disconnect after 15 minutes. (just survey the congregation whenever you're sitting in church sometime...) Since a typical class period is 50 minutes, two teaching segments per hour with 5 minute prep pads is 20 minutes. Yes, I'm very analytic in that regard. However, no, it doesn't mean that 20 minutes is a hard fast maxim; but it does seem to work well with adult learners - an instructors as well. -R