Angular SHM follows $\tau = -C\theta$ where $C$ is the torsional constant and $\theta$ is the angular displacement. The equation of motion is $I\ddot{\theta} = -C\theta$, giving $\omega = \sqrt{C/I}$ and $T = 2\pi\sqrt{I/C}$. For a physical (compound) pendulum of mass $m$ with COM at distance $d$ from pivot: $T = 2\pi\sqrt{I/(mgd)}$, where $I$ is the moment of inertia about the pivot. The equivalent simple pendulum length is $l_{\text{eq}} = I/(md)$.