In my speech classes, I give what seems like a simple assignment: Prepare a three-minute speech on any topic that interests you. That's only enough time for about 450 words. When these speeches are delivered, almost no one finishes in the allotted time.
The problem is that most people have a very unrealistic idea of what they can say in a short period of time. And if people overload three minutes to such an extent, imagine what they will do with a 20-minute speech! Everyone tries to fit in too much or tries to talk faster as they realize they are running out of time. This time trap points out a key to a powerful speech or presentation: Practice—and time—your delivery. Even though the only way to really see if your speech fits a time limit is to practice it, here are some guidelines for length you can use before you write: If your speech should be 20 minutes, figure on covering four major points; 30 minutes leaves you room for five points; and a one-hour speech allows you to work in eight major points.
After your speech is written, you can check for time by counting words. We speak at approximately 150 words per minute. Therefore, if you count the number of words in your speech, and add a minute or two for audience reaction, you should come up with a rough estimate of how long your speech will be. Of course, the best way to estimate is to time yourself while you practice, but counting words can let you know if you're way over (or way under) before you even begin.
These limits make it sound like you don't have a lot of time and room—and you don't. It's staying power that you're after, and that means limiting yourself and choosing your main points with care.