Whenever a couple ask me how long my wedding ceremonies are I can always give the same easy answer: about 18 minutes.
TED Talks curator Chris Anderson, limits talks to 18 minutes because:
18 minutes is short enough to hold people's attention, including on the Internet, and precise enough to be taken seriously. But it's also long enough to say something that matters.
Some of ceremonies are longer, some shorter, but in mind is the fact that we are creating something seriously important, whilst holding a crowd's attention, plus having fun at the same time - so the goal is 18 minutes.
More important things can be communicated in less.