TL;DR: NIOSH would recommend limiting the 8 hour exposure to less than 85 dBA. At 100 dBA, NIOSH recommends less than 15 minutes of exposure per day. NIOSH also recommends a 3 dBA exchange rate so that every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time.
15 minutes is 7 doublings away from 32 hours, i.e. constant exposure. So, 100-(7*3)= 79dBA is safe, and 82dBA is safe for long-term exposure, and 85dBA is a limit at which you should have hearing protection.
None of that comment seemed unreasonable to me - I've had large over-ear sets of headphones that require me to turn the volume up pretty high. It takes more to drive them, that's why some people own headphone amps.
Compare that to IEM's like the Sennheiser cx 300 - 5% of max volume at my computer might be too loud.
And it would surprise me if headphone usage at reasonable volumes is worse than going to loud concerts, even if you use headphones for long periods of time. If I'm at a concert or a loud bar for an hour or two, my ears feel really fatigued afterwards, everything seems quieter. I've never turned up headphones loud enough to experience that.
TL;DR "The sensitivity of headphones is usually between about 80 and 125 dB/mW and usually measured at 1 kHz." and note that since dB is logarithmic, every 6dB is 2x the energy (human-perceived "doubling of volume" is typically ~10dB; see also: https://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves )
Sure. I added some reading material links to my comment. The Rane document has an OSHA safe sound levels table at the end, a good reference. Keep in mind that the dB scale is logarithmic if you want to crunch numbers on device power, headphone sensitivity and impedance, and resulting SPLs.
https://www.osha.gov/SLTC/noisehearingconservation/
TL;DR: NIOSH would recommend limiting the 8 hour exposure to less than 85 dBA. At 100 dBA, NIOSH recommends less than 15 minutes of exposure per day. NIOSH also recommends a 3 dBA exchange rate so that every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time.
15 minutes is 7 doublings away from 32 hours, i.e. constant exposure. So, 100-(7*3)= 79dBA is safe, and 82dBA is safe for long-term exposure, and 85dBA is a limit at which you should have hearing protection.