  We investigate three-dimensional ideal hydrodynamic evolution, with Landau
initial conditions, incorporating event-by-event variation with many events and
transverse density inhomogeneities. We show that the transition to
boost-invariant flow occurs too late for realistic setups, with corrections of
$\order{20-30\%}$ expected at freezeout for most scenarios. Moreover, the
deviation from boost-invariance is correlated with both transverse flow and
elliptic flow, with the more highly transversely flowing regions also showing
the most violation of boost invariance. Therefore, if longitudinal flow is not
fully developed at the early stages of heavy ion collisions, hydrodynamics
where boost-invariance holds at mid-rapidity is inadequate to extract transport
coefficients of the quark-gluon plasma. We conclude by arguing that developing
experimental probes of boost invariance is necessary, and suggest some
promising directions in this regard.
