“How long should my presentation be?”

It’s one of the most common presentation questions I’m asked by sales and marketing professionals as well as CEOs and business leaders. I won’t offer the “right” answer, but I’ll give you a good one. There are few right answers because communication is a gray area. You can always find an exception. But there are good answers based on scientific and anecdotal evidence. Here’s a good one.

The good answer is 15 minutes. A speaker can accomplish a lot in that time frame. John F. Kennedy galvanized a nation in 15 minutes. Steve Jobs took 15 minutes to inspire a generation of students in his 2005 commencement speech at Stanford University. Facebook COO Sheryl Sandberg launched “Lean In” in this presentation which came in at 14 minutes and 50 seconds. If 15 minutes is enough time to launch a movement, why is your pitch dragging on for much longer?

John Medina is a molecular biologist at the University of Washington. His research has led him to conclude that—given a presentation of average interest—most people tune out after 10 minutes. “People don’t pay attention to boring things,” he says. Medina believes you have 9 minutes and 59 seconds to grab and keep a person’s attention. After that, the brain needs a break. Medina does not imply that a presentation should only last 10 minutes (his classes are longer than that, after all), but he does suggest that a speaker change up the presentation at the 10-minute mark and tell a story, show video, conduct a demo, etc. Once that’s done, the presentation goes on. How long? I suggest another five minutes before turning to your audience for questions.

Fifteen minutes is plenty of time to pitch a product, service, or new idea.

There’s something inherently appealing about 15 minutes. Insurance companies are among the most data-driven companies in the world and use advanced analytics to examine the cost of risk. One insurance brand, GEICO, did the math and determined that consumers would be willing to spend 15 minutes to save 15% on car insurance. In the same way, the average listener can focus on a reasonably interesting 15 minute presentation, but any longer might result in “cognitive overload,” delivering too much information at once.

“But what about TED Talks, Carmine? Those are 18 minutes.”

It’s true that the maximum length of a TED talk is 18 minutes. According to TED curator Chris Anderson, the length is “short enough to hold people’s attention and precise enough to be taken seriously. But it’s also long enough to say something that matters.”

Here’s the twist. Give the average business professional 15 minutes and they’ll almost always take a little longer. I recommend shooting for 15 minutes, and if you go three minutes longer, you’ll still be within an acceptable time frame, a time frame that leaves your audience satisfied without being bored. If your listener, client, or prospect feels the need to have a discussion for the next 40 minutes, that’s fine. You’ve piqued their interest and they want to learn more. It’s always better to leave an audience wanting more than wanting to search for the nearest exit.

Read article on Forbes.com