When somebody is introducing you when you are about to speak, how long should your introduction be?
The worst thing you can do is not give the introducer your introduction and they just go to your website and hit Print on the most boring, long, and tedious bio which takes them 3 minutes to read when you are reading for only 6 minutes. As a result, they up using half of your time to just introduce you.
You always want to create a special introduction just for the organization you are speaking to, the event, keeping in mind your audience and how you can tell them why you were brought there and why your expertise is relevant.
If you are going to speak for 5 minutes, then your introduction should not be more than 20 or 30 seconds long. If you are the main keynote speaker and people have flown in to hear you speak at a conference and you are speaking for 90 minutes, then you can stretch it up to 2 to 3 minutes. But even then, you must keep it brief and relevant. The audience does not need to know where you went to high school or what your second Masters is. Tell them what they will find relevant and what will make them want to listen to you more.
Do that, and you will certainly be in great shape.