  We show that, in ideal-spin hydrodynamics, the components of the spin tensor
follow damped wave equations. The damping rate is related to nonlocal
collisions of the particles in the fluid, which enter at first order in $\hbar$
in a semi-classical expansion. This rate provides an estimate for the timescale
of spin equilibration and is computed by considering a system of spin-1/2
fermions subject to a quartic self-interaction. It is found that the relaxation
times of the components of the spin tensor can become very large compared to
the usual dissipative timescales of the system. Our results suggest that the
spin degrees of freedom in a heavy-ion collision may not be in equilibrium by
the time of freeze-out, and thus should be treated dynamically.
