I think it's actually dependent on how big the other ship is. Have a look at an example of two spacecraft approching ech other in space:
http://www.youtube.com/watch?v=MhISvjoc6Gk
In this case, Eagle, the LEM of Apollo 11, is returning from the surface of the Moon to the command module Columbia. I assume that Columbia is no more than 5km or so above the surface. At 48 seconds into the video, the LEM is invisible.
Assuming some putative starship like Enterprise is about 10 city blocks long, I would expect it would be a faint dot barely visible to the eye if it were more than 100km away. Of course, if it were a Klingon warship, with cloaking or even just black housepaint on the outside, you'd be getting a photon torpedo up your keester before you even knew it was there.