Depends on the size of their pupil and sensitivity of their eye equivalent features.. if they have a telescope then 5 billion light years because that’s how old the sun is… if their pupils were massive I believe they could technically see it with the naked eye but it would be incredibly tiny.. they just need to capture enough light from the source to be visible

You can provide a rough outline but the further you go out there are exponentially more variables and some of which you can never account for

Future predictions are always wrong because you cannot account for truly new technologies and the scope of view is often too small to capture global events, impacts of other inventions, and impacts of other disciplines.. hell you could argued 20 years ago that people today wouldn’t be having any kind of body issue thanks to stem cells.. well then the religions nuts got involved, swayed their party and manipulated the information so it seems bad and viola minimal progress has been made in 20 years compared to what it could’ve been

I mean they proved that a donut shaped planet could exist so I’m positive this can exist.. space is essentially the infinite so if the physics are possible, it’s probably out there