What formula is used to figure the time it takes a radio signal to travel from Mars to Earth?

Radio waves, like all other waveforms in the electromagnetic spectrum travel at what is known as the speed of light. i.e. 3x10 to the power 8 metres per second (300,000,000)

but the distance between the earth and mars varies enormously depending on their relative positions in their orbits around the sun. Anything from approx 35 million miles at it's nearest to about 340 million miles or thereabouts at it's furthest.

So roughly 3 minutes minimum to about 30 minutes maximum.