It seems like an innocent question. Unfortunately, reality and experience suggest otherwise.
My general rule of thumb on speech length is to write (as a first draft) 135 words per delivered minute. Of course, exceptions abound.
In general, this 135 rule will get you in the ballpark. For a 20-minute speech, shoot for 2,700 words. For a 15-minute speech, shoot for 2,025 or somewhere around 2,000.
Problems arise on two big fronts.
First, not every speaker is the same. I used to write for an executive from Mississippi. He was a great man and leader who used to routinely deliver 89.5 words per minute. Some of that was his deliberate southern speech, but he also liked to ad-lib.
I learned to build in those factors and adjust my copy. If I didn’t, he would exceed his time limit, which has the potential to antagonize an audience.
When a speaker goes beyond his allotted time on a conference agenda, it not only frustrates the audience, but makes it harder for them to hear the speaker’s message—especially the close.
They already built in a “the speaker will stop here” mindset, and when the speaker goes past that—when he violates that audience expectation—the audience tends to tune out.