The "Demo video for Aperture" will take a few minutes I guess. I don't know how long his keynotes usually take, I'm thinkin' they're brief, maybe not. I've never seen him speak in person. I have seen Woz, who is more interesting to listen to. ;')
Previous Jobs' keynotes have run around 1 hour to 1:30.
http://www.macrumorslive.com/
10:41 am Shows pic of Jobs and Woz. will be 30 years in 4-1-2006.