I stopped wearing headphones for the most part since I work out of an office with isolation, but when I have to work around others or noise, my headphones are on way to loud likely. I use it to drown out the exterior sounds so all I hear is my music and my thoughts. For the most part vocals for me will fade into the background and I am more listening for the beat to keep me focused and undistracted.
But as for volume, I don't know a number but it is generally loud enough that if I take off my headphones you can still hear the music clearly a couple of feet away. btw -- I wear mid-grade Sennheiser over the ear headphones.
Thankfully I don't have to do that too much anymore so I can just have background music on in my office, but when I used to work in the "open" environment I couldn't function productively without headphones and music.