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.