it depends a lot on the type of radio, resistance to interferance, frequency and area you fly in.

In Aus (36Mhz band) the latest FM duel conversion set ups used in flat country areas (less interferance) are good for about 3 miles.

basicaly there a LOS (line of sight) set up if you can't see it (distance, trees, hills, buildings) you cant control it, and even if you can whats the point if you ccant see it.

at the other end of the scale is the park fly type AM and low power FM set ups, in some cases there only good for 500 yards and less in some cases (special super light indoor gear)