In an earlier edition there was a table that listed the maximum distances things could be spotted according to their size, contrast, and distance. Some things far away could be seen but few details could be made out. For example: (note that these numbers are made up and are not from this half-remembered table from a [I think] 2e book)
A Medium humanoid moving at 1000 feet is visible, but is it a human, gnoll, orc, bugbear, dwarf, elf, etc? You can't tell.
A Medium creature moving at 1500 feet, you can't tell its classification (humanoid, ooze, dragon, etc.) but you can tell some creature is there (just a moving dot on the horizon, but visible). You might could attack this creature with arrows or spells, but not knowing what it was, would you?
A castle, a keep, and a city can be seen for far longer distances.
The sun, the moon, the stars are special cases because they are extremely large and they provide their own light. A black hole is enormous but who has seen one with the naked eye? Even one closer and larger than some stars.
Alas, contrast was dropped for 3.xe to simplify things and for Spot, size is just hinted at with no size mods given for this case (which should we use: Hide, Grapple, or AC/Attack size mods).
Nevertheless, a maximum distance for Spotting something is given in the rules for each type of terrain. For example, plains is:
SRD said:
In plains terrain, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 6d6×40 feet, although the specifics of your map may restrict line of sight.
So, 1,440 feet in plains terrain (less is more likely). This upper limit depends less on individual stats and skills than on the terrain, it seems.
Also, with darkvision, it can also be modified by a turn in the corridor ahead, thus limiting your 60 ft. darkvision to only 10 feet. Thus a monster 15 feet away is not visible to you despite your 60 ft. unwavering range. Thus with spells, PrCs, magic items, and specific conditions Darkvision range is variable, too without using the telescope.
Ciao
Dave