The farthest a CB radio can reach is largely dependent on the power output of the radio, the height and location of the antenna, and the surrounding terrain.
CB radios are limited by the FCC to a maximum power output of 4 watts for mobile units and 5 watts for base stations. With a maximum power output, a mobile unit will have a maximum range of about 3-5 miles in ideal conditions, such as flat terrain and no obstructions. Base stations have a slightly higher range due to the more powerful transmitter, and the ability to have taller antennas.
However, in real-world conditions, the range of a CB radio can be significantly less due to the presence of obstructions such as buildings, hills, and trees, and interference from other electronic devices. The range can also be affected by the type and quality of the antenna used.
It is also worth noting that CB radios can also be used to communicate over longer distances using skip propagation, which is a form of radio propagation that allows radio waves to travel long distances by bouncing off the ionosphere. In this way, a CB radio signal can travel much farther than the normal line-of-sight range, but it is highly dependent on the ionosphere conditions.
In summary, the range of a CB radio can vary greatly depending on the conditions, but generally, a range of 3-5 miles can be expected in ideal conditions with a mobile unit and slightly higher with a base station.