For any RF system, the golden rule is Signal-to-Noise (S/N) ratio, which as CHING said depends on power and distance.
Generally speaking, for RC cars, you only need a distance that allows you to see and control the vehicle / boat / plane, anything beyond this is (practically) useless.
One note about 2.4GHZ band, it is a license free band and thus shared by many standards, including Bluetooth, WiFi, Microwave Oven and so on, as well as your 2.4 RC gears. Signals from other standards are noises to another. The implication is that if you use your RC gears with another RF system around, like WiFi (esp the new 802.11n wide band), the noise being 'see' by the RC system is greater. Depends on the distance and power, it may give control or pairing troubles, esp if you are next to a powerful WiFi router or working Microwave oven.