Depends. My IEMs have higher impedance than your usual pair of headphones, and then it also depends on what I'm listening to from my self-ripped .flac library. Right now I am listening to Savage Amusement by Scorpions and I can listen to it enjoyably at 65 out of 120 on my Walkman. The Dark Side of the Moon by Pink Floyd is a tad bit quieter so on that album I have 75. Usually it's around this volume range, but sometimes if it's a raw grimey death metal album I have to go even higher.

tl;dr:

It mostly depends on how loud or quiet the album I'm listening to is.