When I get hired as a keynote speaker, a common question from the meeting planner is, "How much time do you need?". My answer is, "How much time do you have?".
When planning a conference agenda, keep these points in mind for speaker time slots:
It depends on the speaker. Some people are painful to listen to for only five minutes, while others can captivate a crowd for hours on end. Ask the speakers how much time they need or hire motivational speakers who can cater the content to the allotted time slot.
It depends on the topic. Any guest speaker would be hard pressed to deliver "1,000 Ways to Audit Your Business Unit" in only 15 minutes. However, a short motivational speech may be just what people need for a quick twenty minute energy booster.
It depends on your schedule. Most meeting planners design schedules and the specific speaking time slots first and than fill in the content and various speakers.
Generally speaking, the shorter the better. A really compelling presentation is usually not a full day speech. A motivational speaker is usually 60 to 90 minutes and a break out sessions can be a little bit longer. If you give speakers too much time, you may find them trying to waste it with irrelevant interaction or activities.
When I was a female motivational speaker for a safety conference recently, the other keynote speaker ended 20 minutes early because he wasn't able to fill his time. It's better he ended the speech early and gave people time to network and visit with vendors than waste people's time with irrelevant content.