Well, usually I optimise 70% jpegs, but it all depends on cropping and colour information in the image too.
The only reason I'm asking is that a mate of mine wants to start up image hosting from his own server and asked me to provide him with all my Flickr uploads. He also wants to know how much space they currently take up.
Without exporting all of the images in one go I've no idea how much space they take up.
Mark