There's no easy guide as it depends on the phones you're wearing but broadly you should aim for the level to be where you can hear a person talking next to you still. If they have to raise their voice for you to hear them then it's probably to loud.
For me though it's about quality versus noise level. A set of phones that lets you hear all of the detail of the game without needing to turn it louder is perfect, otherwise you feel like you need the extra volume. And if it's a game where you need to hear your surroundings to get an advantage it also helps your playing too.