I found this an interesting read, and there is a decent app that will calculate listening levels to give you and idea of the level at which you listen to your music (I realize there are other variables involved though, like the level at which your music was recored at, and etc... that are not accounted for). http://www.ifans.com/forums/threads/estimating-headphone-volume.364522/
Protect your hearing!
Protect your hearing!