TED presentations take a bit less than 20 minutes. If you have watched a few of these presentations, you will have noticed that this time is more than enough to get a complex idea across in your presentation.
Apparently, 20 minutes is also the average time a grown up can really focus his attention (source).
Add these two observations up and you realize that you need to design your presentation in such a way that the full pitch comes across in roughly this time. Anything more can be added as additional case studies examples, other plot variations. Possibly again in blocks of 10-20 minutes.
A few people with a lot more knowledge than I have about attention span and the brain are reading this blog, feel free to add your perspective.