> "As a rule of thumb, you should only use MP3 devices at levels up to 60% of maximum volume for a total of 60 minutes a day" (http://www.osteopathic.org/osteopathic-health/about-your-hea...).

Out of curiosity, is there a peer-reviewed study on this? Moderation sounds good, but I'd like to understand at what intensity (dB) for what period could potentially constitute hearing loss.

The 60% cited in that link seems to be based on a 120dB max (so: 72dB), which is a fair bit higher than most consumer devices (with consumer headphone impedance levels). Some cursory research shows that it's a bit closer to 103 - 109dB for an iPhone (and similar devices), which puts it around 63dB.

For context, I use canalphones for the isolation, and so I can keep the output down. I also understand that these aren't viable and/or comfortable for everyone.

TL;DR: How long can someone sustain ~63dB without potential hearing damage?