If by presentation, you mean a speech (not training or workshop) then the ideal length is between 15 and 20 minutes. In the 1970s the U.S. Navy conducted a study to find out how long people can listen to other people talk.
The Navy wanted to find out how to better use the time of instructors and students in the Navy's education system. What they discovered about the ideal length for a presentation surprised a lot of people. It wasn't an hour -- or even a half hour.
The Navy found an audience's ability to focus on what the speaker is saying and then remember what was said drops off significantly at 18 minutes. They found this to be true across the board -- in classroom, presentation, or lecture environments.