The average attention span of an adult is said to be about 20 minutes. That's 20 minutes of sustained attention, during which people can - more or less - concentrate on a specific task or, in our context, a presentation. The "20" is not be taken as an exact number, of course, but more like a rule of thumb. Methods like the Pomodoro Technique, which uses 25-minute intervals of focussed work would also be covered by this.
However, as Dr. John Medina argues, it's probably a good idea not to rely on your audience's undivided attention to last the full 20 minutes. Instead, he proposes a 10-minute rule (again, the "10" is not to be taken as an exact number). Dr. Medina suggests that you change your approach every 8-12 minutes or so. By this he means to switch between simply showing slides to showing a video, drawing on a whiteboard, or doing an exercise.
Exercises or other forms of audience interaction have the added advantage of helping your audience to better remember the content of your presentation. Tell them about something, then give them a (simple) exercise to use it in practice. "Learning by doing" is an old and well-tested approach that you can make use of in your presentation. Even a simple show of hands will make your audience switch from passively consuming your content to having to think about it and making a decision.
You may think audience interaction is only really possible in longer talks. You'd think you would have to explain things first, then set the context for an exercise and give the audience time to do it. But interaction and even exercises don't have to be time consuming. A show of hands after a question only takes a few seconds. Simple exercises can last about 2 minutes to discuss something with the person sitting next to you. These are things you can easily integrate even if you only have 15 minutes (or less) for your presentation. Plus, as mentioned above, they are more effective in helping your audience understand and remember your content, since it forces them to do something with their newly acquired knowledge while it's still fresh in their minds.
I think there are 3 common misconceptions or fears that stop presenters from building interactivity into their presentations:
1. Nobody's going to participate
It's true that some people will be reluctant to engage in a conversation with a random stranger. But they will see that everybody else is doing it and will comply. I think the reluctance is more often coming from the novelty of such an activity. People are used to just sit there for an hour and listen. In my experience, once they overcome their reluctance, they will actually enjoy it and remember the experience fondly.
2. I'm going to lose control of my audience
Agreed, it can be hard to make yourself heard over the noise of 50 people chatting in a small room. But your audience will accept that you are in control of the presentation (they did comply with doing the exercise after all). It's really only a matter of letting them know that the exercise is over and they will quickly sit down again and be quiet. Try using an acoustic signal (I use a gong signal tone on my mobile phone, with the volume set to maximum) and plan with an extra minute for things to settle down.
3. They will want to discuss their findings
We are used to the idea that an exercise will produce results and that we will then have to present and explain our findings. You can of course do that with your audience when you have enough time. It's perfectly okay, however, to summarise the results for them. I.e. you can prepare the exercise such that you know the outcome and present (with slides, if you must) the expected results. You can always, before moving on, check with the audience if they found something that you hadn't discussed or anticipated.
Building interactivity into your presentation will bring some energy (back) into the room and your audience. By making them think about what you just told them, they will remember and understand things better. Exercises don't have to take a lot of time, so given the benefits, there's no excuse not to integrate them into your next presentation.
(Photo from the 2012 Presentation Zen seminars in London by yours truly)
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