how do you solve for time using the radius of a circle and gravity?
You do not use gravity at all.
You may use the radius of the earth depending on the question.
You might also like to investigate Kepler's 3 Laws of Planetary motions:
Johannes Kepler, working with data painstakingly collected by Tycho Brahe without the aid of a telescope, developed three laws which described the motion of the planets across the sky.
1. The Law of Orbits: All planets move in elliptical orbits, with the sun at one focus.
2. The Law of Areas: A line that connects a planet to the sun sweeps out equal areas in equal times.
3. The Law of Periods: The square of the period of any planet is proportional to the cube of the semimajor axis of its orbit.
Kepler's laws were derived for orbits around the sun, but they apply to satellite orbits as well.