There's no precise criteria but the usual measure is ppd (pixels per degree) and it needs to be high enough such that detailed content (such as text) displayed at a reasonable size is clearly legible without eye strain.

> "Could you not just move your face closer to the virtual screen to see finer details?"

Sure, but then you have the problem of, say, using an IMAX screen as your computer monitor. The level of head motion required to consume screen content (i.e., a ton of large head movements) would make the device very uncomfortable quite quickly.

The Vision Pro has about ~35ppd and generally people seems to think it hits the bar for monitor replacement. Meta Quest 3 has ~25ppd and generally people seem to think it does not. The Steam Frame is specs-wise much closer to Quest 3 than Vision Pro.

There are some software things you can do to increase legibility of details like text, but ultimately you do need physical pixels.

Even the vision pro at 35ppd simply isn't close to the PPD you can get from a good desktop monitor (we can calculate PPD for desktop monitors too, using size and viewing distance).

Apple's "retina" HiDPI monitors typically have PPD well beyond 35 at ordinary viewing distances, even a 1080p 24 inch monitor on your desk can exceed this.

For me personally, 35ppd feels about the minimum I would accept for emulating a monitor for text work in a VR headset, but it's still not good enough for me to even begin thinking about using it to replace any of my monitors.

> https://phrogz.net/tmp/ScreenDensityCalculator.html