Radio waves are a part of the electromagnetic spectrum, and if transmitted using an omnidirectional antenna, they are subject to the inverse square law, which shows how the initial signal strength (or light intensity, if the source is a point of light) decreases with distance.
To get an exact answer, it is necessary to know (1) if the signal is directional (such as sent via a parabolic dish in a certain direction, in which case it will attenuate at a lower rate) or omnidirectional; and (2) the initial signal strength. Regular radio transmissions are omnidirectional.
See also this question which mentions Voyager, the most distant probe sent by NASA, and antenna required to detect the signal, though note that the Voyager probes use a directional dish antenna to transmit the signal, and the Voyager 1 signal requires 22 hours and 35 minutes to reach Earth at this time.
The Deep Space Network dish antennas receiving the signal are 70 meters in diameter, and the dish antenna on the spacecraft is 3.7 meters in diameter.
The largest radio telescope is 500 meters in diameter.
Regarding the signal strength issue, see this question which points out that radio signals are also photons, just like those emitted by stars. On the night sky only stars that are a few hundred light years distant can be seen easily - although each star is a far more powerful electromagnetic transmitter than an artificial transmitter.
It may also be worth considering that radio transmissions such as those sent out in space from Earth, are likely sent only for a certain time period in the development of a human civilization, probably a few centuries, until a more appropriate technology is discovered, because radio signals, lasers, and the electromagnetic spectrum in general, is not useful for communication at interstellar distances.
To illustrate, the closest star system to our sun is Alpha Centauri, at 4.3 light years distance - so if an email is sent there by radio signal, the reply would arrive at the earliest after 8.6 years (round trip time).
Our galaxy is 120,000 light years in diameter, so any electromagnetic signal received from a distant planet, would be from its distant past as well - and if the window of time where the people on that planet used primitive radio signal technology - has already passed the Earth, like a wave crest, and they are now using something more advanced which we do not detect, then we hear nothing, even if a large enough radio telescope dish would exist.
So a dish would have to be very large to receive a radio signal only from the nearest stars, and would have to be located in space, shielded from the sun by a planet, or even outside our solar system, and even so, this would work only if the humans living on planets around nearby stars have not yet passed the radio signal stage in their technological development.
This would be like a civilization trying to contact us using smoke signals, or looking for our smoke signals, and receiving no reply/sign of life, because we use radio signals, not smoke signals.