921 words got me 6 minutes and 2 seconds, but as pLuvia and Riviet have said, it depends on how fast you read.
when i practice my speeches, i record myself saying them (even though that might sound a bit weird) listen to it and practice the pace and pauses that work within the time frame. (hope that made sense... it works for me)
just a note; make sure you time yourself reading the speech out loud, this may sound stupid but i tried to time a speech while i read it im my head, thinking it would be the same. it wasnt.