For a typical person, a safe volume level for listening to music with headphones is between 10 and 60 percent of the maximum volume on a portable music player. Levels higher than that put a listener at risk for permanent hearing damage.
Different types of headphones allow people to safely listen to music at higher levels. Supra-Aural earphones, which go over the ear, are generally safe at 70 percent of the maximum volume for up to 20 hours a day, whereas typical stock earphones are only safe for about 4.5 hours at that same volume level. A good way to find a safe listening level using headphones is to lower the audio level until it's barely audible, and then raise it back up by a few notches.