There is a lot of debate over this generic advice. Many people advocate for short presentations because people are losing their attention spans. But wait a second: I have seen speakers get up, speak very vaguely, be very abstract, dump a bunch of data on the audience, and nobody remembered it after two minutes.
On the other hand, I have seen speakers stand up to speak in the morning and speak until midnight and the audience still wants more because they were interesting, engaging, and memorable with lots of stories and anecdotes that related and interacted with the audience. So it is not necessarily about speaking for a shorter period, it is about how interesting you are.
A study done by the United States Department of Defense in 1974 showed that the optimum length of a speech was 17 and a half minutes long. Well, i do not agree. If you are boring for 30 seconds, it is too long and if you are interesting for two hours, it can be too short.
You need to step back for a moment and ask yourself what you goal is.what messages do you want the audience to understand and remember in order to get them to do what you want them to do? Then work backwards from there. If you have been given an hour long slot, prepare a 45 minute long speech and leave the rest for questions. You must have heard about the time limit of 18 minutes at the TED talks. There is no magic behind it- it just forces smart speakers to redefine and narrow it down to their best points only.
So make sure you take enough time to make your point in an interesting fashion- no more than that but no less either.