On an average computer this can be as much as 30GB. You can just go into the Spotify cache folder and delete the information though (not recommended unless space on your computer is needed for other files) but this does mean that when you come to listen to the same song again, which you probably will, as you liked the song you were listening to, you will have to retrieve the song information again, and won't be able to help other users out by sending them song data to allow them to listen to the song faster.