Remember - if you have ringing in your ears or dull hearing after listening to your MP3 or personal stereo, it is a serious warning that the music was loud enough to damage your hearing.

There is no simple answer to this question as it depends on the make and model of music player, the type of headphones you are using and on your susceptibility to hearing damage. As a rule of thumb, if the music is uncomfortable for you to listen to then it's too loud, or if you can't hear external sounds when you've got your headphones on, again, it's probably too loud.
But that is not the complete picture, because noise damage is caused by two factors - the volume you listen at and how long you listen for. When you vary the volume, the length of time you can ‘safely’ listen for changes too. Simply put, the higher the volume the shorter time you should listen for.
Apple have volume limiting software which you can download onto your iPod. The software lets you set a maximum limit to prevent the volume exceeding the level you set.
Microsoft have provided lots of hearing safety advice on the Zune website. If you've recently bought a Zune portable media player, take a look at their advice.