The time period of a simple pendulum of length $L$ in a gravitational field $g$ is:

For small oscillations the pendulum's equation of motion reduces to SHM with $\omega = \sqrt{g/L}$. So period $T = 2\pi/\omega = 2\pi\sqrt{L/g}$. Independent of the mass of the bob (a key contrast with the spring-mass system, where $T \propto \sqrt{m}$). Note: only valid for small-amplitude swings. At larger amplitudes the small-angle approximation fails and the period depends slightly on amplitude.