Kepler's third law is all about the timing of the planets' orbits. It states, "the ratio between the square of the time needed by a planet to make a revolution around the sun and the cube of its average distance from the sun is the same for all planets." This law can be expressed in the following formula:
(Period of planet)^2/(Average orbit radius)^3 = Same result for all planets.
You can use this formula to collect data for your entry into a science fair competition