How long would it take a radio wave to travel from Venus to Earth?
One major problem with intrasolar and interstellar communication is the vast distances involved. On Earth, radio communication is all-but instantaneous, because radio waves travel at the speed of light, approximately 300,000 kilometres per second. A good real-world example of radio lag in space is the roughly three-minute lag between Earth and the Moon; when NASA communicated with astronauts on the Moon's surface, replies took about 1.5 minutes for the original broadcast to reach the Moon, and another 1.5 minutes for the reply.
With that said, calculating the radio delay between Earth and Venus -- the closest planet to Earth -- is now simply a math problem. However, the distance between Venus and Earth varies throughout the year of each; at its closest, Venus is about 38 million km from Earth, and about 261 million km at its farthest. Radio communication, therefore, would probably be done through a system of satellites positioned in such a way that one or more would always be in line-of-sight with both planets.
If each distance extreme were to occur with line-of-sight, the breakdown of time would be as follows:
Closest: 38,000,000 km / 300,000 km/s = 126.67 seconds, or about 2.11 minutes.
Farthest: 261,000,000 / 300,000 km/s = 870 seconds, or about 14 minutes.
Average: 149,500,000 / 300,000 km/s = 498.3 seconds, or about 8.3 minutes.
These numbers will change depending on solar conditions, positions of the planets, and natural degradation in radio signal over long distances and through atmospheric conditions.