Solution
= Solution
Without friction, <rotational kinetic energy> is conserved:
$$
K_0=\frac12I\omega^2.
$$
The constant angular speed is $\omega=\sqrt{2K_0/I}$, so one full rotation takes
$$
\boxed{T=\frac{2\pi}{\omega}
=2\pi\sqrt{\frac{I}{2K_0}}}.
$$