  We simulate the space-time dynamics of high-energy collisions based on a
microscopic kinetic description, 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. By determining the breakdown of hydrodynamics as a function of system size
and energy, we find that it is quantitatively accurate in central lead-lead
collisions at LHC energies, but should not be used in typical proton-lead or
proton-proton collisions, where the development of collective flow cannot
accurately be described within hydrodynamics.
