The simple answer is 80-150 words per minute, if you are reading aloud from a script and never pause or look up at your audience or ad lib. But -- that is the worst possible way to do a speech.
When I speak in public (giving invited lectures, presentations at conferences, etc.) I will mark a few places in the text to insert extemporaneous comments, often ones that inject some humour into the speech. It is also good to plan for pauses to make eye contact with your audience and let important points sink in. The absolute worst thing most novice speakers do is talk too fast and forget to pause.
For timing, you should practice your final draft with a stop watch at least 2-3 times before you give it, re-phrasing any pieces that sound awkward when spoken. As speeches normally take a bit longer in a real setting than when you are practicing in your room, aim for 7 to 7.5 minutes in practice to get an 8 minute speech.