  We derive relations for the decay of the kinetic and magnetic energies and
the growth of the Taylor and integral scales in unforced, incompressible,
homogeneous and isotropic three-dimensional magnetohydrodynamic (3DMHD)
turbulence with power-law initial energy spectra. We also derive bounds for the
decay of the cross- and magnetic helicities. We then present results from
systematic numerical studies of such decay both within the context of an MHD
shell model and direct numerical simulations (DNS) of 3DMHD. We show explicitly
that our results about the power-law decay of the energies hold for times
$t<t_*$, where $t_*$ is the time at which the integral scales become comparable
to the system size. For $t<t_*$, our numerical results are consistent with
those predicted by the principle of `permanence of large eddies'.
