The short answer to this question is, of course, that it depends. It depends on your audience, on your content, on the setting. Often the choice is taken entirely out of your hands, for example if you are allocated a predetermined speaking slot.
So how do you make this decision? If you do have the choice to make as to how long your presentation should be, there are a few factors to take into consideration.
Ultimately, however, this should all come down to attention span. Studies show that the average attention span for an audience is twenty minutes. While this can vary depending on subject matter and initial interest levels of audience, it is a good basis to follow.
Of course, you can’t always limit your presentation to twenty minutes. So how do you get around this in a longer setting? What we would advise presenters to do in this situation is to hold breaks – hard or soft breaks, depending on what’s appropriate – at twenty minute intervals.
A soft break is anything that stops the audience focusing on paying attention for a few minutes and encourages them to ‘wake up’ and engage in some mental activity. A great way to do this is to ask a question, or pose a quick puzzle for the audience to solve. This gets the audience to actively engage with you and breaks the monotony of just sitting there and listening.
A hard break is typically a coffee or lunch break. This is particularly appropriate in a training setting. Audiences can get by on one or two soft breaks in a long presentation, but after a while they’ll need a full hard break to re-engage.
Finally, presenters should always remember that how long a presentation should be in minutes is not the same thing as how long it should be in the number of slides. Don’t worry so much about slide number and instead focus on time and audience engagement levels.