Many people who use MP3 devices experience tinnitus (ringing in the ears) and even some temporary hearing decrease. Both these are symptoms that your hearing is at risk. Loud music is proven to cause significant hearing loss over time. Louder music and sounds will cause the hearing loss process to speed up rapidly. Case in point: I had a teenager come in for a hearing test and he already has a mild to moderate permanent hearing loss at one of the frequencies in both ears. Upon counseling him, he admitted to using his MP3 player more than six hours a day and demonstrated how loud he likes to play it. I explained that if someone sitting next to you can hear the music as well, then it is probably loud enough to cause hearing loss for the wearer. Note that your auditory system does not distinguish rock from classical music, so the hearing loss will be the same for any type of music, if played loud enough.
The goal is to preserve your hearing by turning the volume down to safe levels. If you are still wondering about what level is safe, I recommend you speak with an audiologist and have a baseline hearing test.