The distance at which you can spot someone boils down (all else being equal) to how much of a range penalty you can soak on your vision roll. There are other modifiers to that Perception roll, of course -- things like darkness, camouflage, or those trees in the way. Being "in plain sight" is a +10, so that makes things much more visible at a distance.
This past thread has some useful discussion.
http://forums.sjgames.com/showthread.php?t=130631
Realistically speaking, you'll also have to consider terrain. Also the horizon distance, if you're talking really long distances and "flat" terrain.
(The horizon on Earth from a 6' / 2m height -- human eyeballs -- is about 3 miles / 5 km. Further than that, and the object is sliding down behind the curve of the planet. Increased height and tall objects to view can greatly increase the horizon distance; you can find calculators on the web it if matters.)