  Despite the theoretical indication that fast neutrino-flavor conversion (FFC)
ubiquitously occurs in core-collapse supernova and binary neutron star merger,
the lack of global simulations has been the greatest obstacle to study their
astrophysical consequences. In this {\it Letter}, we present large-scale ($50
{\rm km}$) simulations of FFC in spherical symmetry by using a novel approach.
We effectively rescale the oscillation scale of FFC by reducing the number of
injected neutrinos in the simulation box, and then extrapolate back to the case
of the target density of neutrinos with a convergence study. We find that FFC
in all models achieves quasi-steady state in the non-linear regime, and its
saturation property of FFC is universal. We also find that temporal- and
spatial variations of FFC are smeared out at large radii due to phase
cancellation through neutrino self-interactions. Finally, we provide a new
diagnostic quantity, ELN-XLN angular crossing, to assess the non-linear
saturation of FFC.
