This all depends on the length of the track, the bitrate of the track and what's on the track. A standard 3 minute, 128 KB/s birate MP3 file is around 3 meg, but this varies depending on what the track is (whether it's loud, quiet has a lot of detail) because MP3 is a lossy compression. You should get around 500 mins of 128 KB/s audio and a 1 GB card will hold twice this. The same goes for JPEG photos as you'll notice that two images with the same dimensions won't be the same size. The higher the detail the bigger the file and this goes for MP3 audio as well.