I find it extremely comfortable. Considerably moreso than with my desk and monitor setup. I'm long sighted and the projection distance means I don't need glasses to see, and they don't get overly hot (nothing compared to a VR headset). Most significantly though, I can change the position of my head whenever I want without changing what I'm looking at, which makes a difference for my neck comfort, posture, etc.

To answer your question directly, I've done multiple 2-3 hour sessions at a time without taking the glasses off and have done 10+ hour days in them when away from home. At home I typically tend to use them for a couple of hours in the afternoons when I'm tired and more willing to sacrifice screen real estate for comfort.

The detail isn't the same and you have to plan your screen layout a bit (i.e. looking at code near the edges of the screen is annoying). But I think they're the future - a much bigger deal than VR or AR.