For those of us that do not want to go deaf before we’re 18 but still want to listen to music, I figured out a rule of thumb:

Hold your headphones/earbuds at arm’s length. If you can distinctly hear the lyrics or instruments, it’s too loud. The limit should be barely hearing the loudest notes.

Yeah, this might not work for people with extra-long or extra-short arms. But it’s always worked for me.