There is a good article here:
It says that the angular resolution of the eye for a line pair is 0.59 arc-minute.
So if you have a display with 300ppi made up from 3 sub-pixels next to each other (R,G,B), and 2 green pixels next to each other, then you will see them as a single green block if you are further away than ~49cm. (unless my trig is failing me quicker than my eyesight, which is likely!)
As a quick test with the diagonal white boxes in the "Android App of the week" advert on the right, I had to be 2m from my monitor before the diagonal lines became straight instead of stair-cases, which is the right distance for 72ppi.
If you are closer than that, then you will see the two green pixels as two green pixels with a black pixel between them. Of course your brain will see all the pixels on the screen like that and may try to ignore it, but I guess that may cause eye-strain or reduced quality.