What's a good length of time for a picture slide show to be?

[Sara A]

I'm throwing a surprise party for my sister and am finding it HARD to narrow down my picture selection. I don't want to make the slideshow too long and bore people, so if you could let me know how low you would want to watch a slideshow about someone else, I would greatly appreciate it. Keep in mind I'll be sure to include photos of people that will be present at the party... I know that helps keep interest.