During an observation, a radio telescope beam points towards a specific point in the sky, identified with its coordinates right ascension and declination. The declination is by definition 0o for the equator, +90o for the celestial north pole and -90o for the celestial south pole. The right ascension runs along the celestial equator.
Telescopes are built to track celestial objects. While tracking a specific point in the sky, the telescope must compensate for the rotation of the Earth, i.e. it follows that point with a speed of 0.25o per minute from east to west.
Therefore, the tracking speed of a radio telescope beam is 0.25o per minute. For sources in the solar system, such as planets and comets, the speed and the trajectory of the telescope beam can differ somewhat from this, because these objects move with respect to the celestial sphere.