Radio SETI

One of the primary sources of radio leakage off of Earth is the carrier waves of television broadcasts. A single TV carrier wave might transmit the equivalent of 300 kW in all directions in a narrow frequency range around 500 MHz.
The two largest radio telescopes in the world are the Arecibo Observatory in Puerto Rico and FAST in Guizhou. They can each detect narrow-band radio signals with a flux as low as ~100 photons/s/km^2.
If there were alien TV broadcasters using carrier waves like we use somewhere in space, how many lightyears could they be away from Earth and in principal be seen by Arecibo or FAST?