Entertainment mogul David Geffen once gave Lynda Obst a key piece of advice: never go into a meeting without an agenda.

He didn’t mean a schedule. He meant that, before you walk into a meeting, you should know what you want to achieve It’s good advice for anyone hoping to make it in Hollywood; it’s even better advice for anyone giving a speech.

The good news is that every invitation to speak at an event comes with its own innate agenda. But that agenda serves the interests of whoever invited you — not your own.

Take timing. Corporate speechwriter Bill Dunne describes a typical staff meeting:

Finally the speaking engagement comes up and gets kicked around, and at some point the CEO asks, “By the way, how long do they want me to talk?” Somebody looks at some notes and comes up with the answer: an hour. Sixty minutes.

Wrong answer.

What the staffer should have said was: “They want an hour but we’ll get them down to a more reasonable time.” The reason that that should be the stock answer is because many if not most speaking opportunities for executives actually do come attached to time slots of one hour or, what’s not much better, 45 minutes.

That’s crazy.

The ideal timing, Dunne suggests — and his experience jibes with mine — is 20 minutes. The ideal timing, that is, for the speaker and the audience.

Consider this. Even with all the tools of spectacle and audiovisual wizardry at their disposal, David Geffen and his colleagues generally don’t figure they can keep today’s movie audiences engaged for much longer than about 100 minutes at a stretch. We’re trying to maintain people’s attention for a fraction of that time, but with an even smaller fraction of those resources. It’s one person and a microphone, and they come stamped with a best-before date of 20 minutes after they open their mouths.

Are there exceptions? Sure, especially if the audience knows you have big news that you’re saving for the very end; watch one of Steve Jobs’ keynotes. If you have a very complex point to make, and you’re a policy wonk speaking to policy wonks, you may be able to hold their attention for an extended speech (but ask yourself if a paper might be a better medium). And if you’re delivering a lot of information that your audience badly wants to hear, you can get away with a longer speech (but again, ask yourself if a speech is the right way to do it).

These cases are rare exceptions, however, especially with invited speakers. So the next time you’re told you have to speak for an hour, don’t be afraid to push back. Look at the invitation as an opening bid, not a take-it-or-leave-it proposition. Here’s Bill Dunne again:

You’re in a position of strength. The last thing a venue manager wants to do is rustle up another speaker after he’s had one in hand. If he seems to recoil from the notion of “only” 20 minutes, it’s usually enough to offer a post-speech Q&A, bringing the presentation to, say, a more ample-sounding 30 minutes. Once you get into that neighborhood, event organizers are usually glad to adjust.

And you — no longer obligated to stoke the clock — can now concentrate on writing or delivering a great speech…