Recently someone asked me: "How much time do you need for your presentation, 30 minutes, 45 minutes?". While I can fill 2 hours with just talking about presentation design I chose to go for the 30 minutes. It focuses you to be to the point and interesting.
TEDTalks can discuss very complex subject matters in just 20 minutes. I have designed 6 minute presentations for startups participating in pitch competitions that managed to convey the entire company's story. Often I find that a time constraint results in a better presentation.