How far can you see something?

I know it seems like a silly question, but...

i find myself asking this question often, especially when thinking about distance scales to use in a new title. so whether its a person, a building, an airship, or a planet, how far can you see something?

it seems to me that there should be some pretty straightforward formula or rule of thumb approximation for:

given an object [sphere] of radius R, how far away must it be before it should no longer be drawn?

keep it generic, in d3d units.

so if my sphere is at: range = 1, it appears to be almost 2R wide. at some distance: range = D, it appears to be about 2R/D wide. correct?

so when D gets big enough that 2R/D is so small its not worth it (given screen rez etc), you should no longer draw it, correct?

and you could plug in your horizontal and vertical resolutions and solve for 2R/D=1 pixel (or < 1 pixel), correct?

does this make sense?

how do YOU figure out what is big enough / close enough to be drawn?