Two weeks. That's the minimum lead time you need for a keynote speech for a client.
Get three weeks if you can. Make it a rule. Whoever does the schedule needs to know: if they get invited to give a keynote speech tomorrow, or two days from now, don't say, "Sure."
This isn't because you're a slow writer. The reason you need at least two weeks is because writing for another person doubles the amount of time it takes to do a draft. If there's a third person involved who has to OK the speech, add another week.
Writing By The Numbers
Here's the math: novelists, reporters and other professional writers may spend all day banging on the keyboard and, at the end of they day, have 500 good words.
Before word processors, great writers like Hemingway were religious about taking paper out of their Underwood typewriters and counting words, by hand. Once they had 500 or 1,000 words -- whatever their daily goal -- they'd stop.
So think about 500 good words a day. A thousand words, if you're lucky, and maybe more if you're completely inspired and hopped up on industrial amounts of caffeine.
Yet there'll be days when you crank all day and wind up deleting it all. An average of 500 is a good guide for planning.
A keynote speech is about 30 minutes. That's 3,000 words.
Divide 3,000 by 500 words a day and you're talking about six working days -- assuming your rough draft is perfect, that you clear your desk and don't attend a single meeting or do any other work, that you don't need a second draft and your boss or client thinks every word is amazing.
This does not happen.
There's always other work, and meetings, and problems with drafts. The rough draft of anything is bad. That's why it's a rough draft.
And it doesn't factor in any time for research, which is essential for a major speech. You can't wing it. Facts and statistics have to be checked and double-checked. New ideas need to be refined and polished.
For your first keynote speech, you'll be happy if it's done after three drafts.
Expect The Unexpected
The toughest part of writing a keynote speech for another person is getting their involvement and OK throughout the process, from the outline to the rough draft to the final draft and practice speeches.
Sometimes, you have to guess. The client or boss is may be of town, stuck in a meeting or otherwise unavailable, and all you know is as the elevator doors closed, they said, "By the way, I need a speech about gorillas for the Rotary Club dinner."
Are they talking about great apes or guys with AK-47s fighting a dictatorship? Are they pro-gorilla or anti-guerilla? Sometimes, you have to guess. That's a nightmare, but it happens.
Bosses and clients are always busy. This is why it's critical to make sure staff, especially whoever does scheduling, knows the right questions to ask of the client and the event organizers. Who exactly is the audience? Will there be other speakers? How long is the speech? Will there be a microphone or a podium?
In the end, the toughest part of writing a keynote speech for another person isn't the actual writing. It's all the other things.