  We simulate the space-time dynamics of high-energy collisions based on a
microscopic kinetic description in the conformal relaxation time approximation,
in order to determine the range of applicability of an effective description in
relativistic viscous hydrodynamics. We find that hydrodynamics provides a
quantitatively accurate description of collective flow when the average inverse
Reynolds number is sufficiently small and the early pre-equilibrium stage is
properly accounted for. We further discuss the implications of our findings for
the (in)applicability of hydrodynamics in proton-proton, proton-nucleus and
light nucleus collisions.
