The period $T$ is the time for one full circuit, in seconds. There are two honest roads to it, and the givens decide which one you walk.

If you know the speed, the geometry road: $v = \dfrac{2\pi r}{T}$ — v equals two pi r over T — where $v$ is the orbital speed in m/s and $r$ the orbital radius in metres; $2\pi r$ is simply one lap's distance, so this says lap distance over lap time. Rearranged for the clock, $T = \dfrac{2\pi r}{v}$. If instead you know the central mass, the gravity road: $T = 2\pi\sqrt{\dfrac{r^3}{GM}}$, which is Kepler's third law in Newton's handwriting — $M$ the central mass in kilograms, and the $r^3$ under the root the reason far orbits are so unhurried.

One discipline, non-negotiable: G thinks in seconds. A period quoted in hours must cross to seconds before it enters a socket, and an answer wanted in hours crosses back at the END, ×3600 one way and ÷3600 the other. Half the lost marks in this lesson are a clock, not a concept.