It's a complex subject, for sure.
It's generally accepted that a continuous noise above 85dB, for 8 hours a day, will result in enough hearing loss over a career to damage the ability to hear and understand speech. The louder the noise, the less time you can be exposed to it without permanent damage. This is the OSHA standard - you can understand your bosses instructions as long as you're working; after that you don't need to hear ...
But most music is not continuously loud, and one recording is not the same loudness another recording at the same setting of the control. And of course, one headphone may be wildly different from another at the same setting of the control. In industrial applications, noise dosimeters are used to evaluate the noise environment, typically in a factory. They measure the cumulative potential damage based on minute-by-minute level readings, and are used to determine whether hearing protection is required, and how much.
Not much of an answer, I'm afraid. You can probably find a cheap sound level meter (Radio Shack used to have one) and, if you have speakers, set it on "A" scale equalization, "slow" response, and play some music, trying to set the level of the peaks to 85dB. Try to remember how loud that sounded. Then you can match that level with earphones using the same recording, and know you are safe - for that particular recording. You'll still need to adjust the level for the next recording though.
