Files are not compressed at a set percentage. We specify a quality rating for video output that will preserve the resolution of a video or picture on the high end of quality for web viewing (there is a certain limit where higher resolution doesn’t look any better on the web due to limitations of browser, monitors, bandwidth, etc.)
Depending on the format of the video, and the complexity of the images and audio in the video, net compression of the file size could vary from 40-80%. 50% or better is a reasonable expectation in most cases.
Photos are another story because we keep the originals for the full view, plus create 2 new images: the optimized slide show one and the thumbnail. So disk usage actually goes up for uploaded photos, not down.
MP3 audio files are not compressed (any further, that is - MP3 is already a compressed format, so we don't perform any additional compression when audio files are uploaded.)