Mix at a level where you can still comfortable hold a conversation with someone next to you.
Everything sounds great loud, but you’ll quickly tire yourself out and lose perspective. In addition, it’s often important to listen at several different levels when mixing. I like to think of the monitor level as similar to a ‘zoom’ control. Sometimes you want to zoom out and see the big picture of how everything fits together. Other time you’ll want to zoom in to focus on the detail of a track. Too much zooming out and you won’t see the details. Too much zooming in and you’ll be overwhelmed and lose perspective.