  We argue that rich star clusters take at least several local dynamical times
to form, and so are quasi-equilibrium structures during their assembly.
Observations supporting this conclusion include morphologies of star-forming
clumps, momentum flux of protostellar outflows from forming clusters, age
spreads of stars in the Orion Nebula Cluster (ONC) and other clusters, and the
age of a dynamical ejection event from the ONC. We show that these long
formation timescales are consistent with the expected star formation rate in
turbulent gas, as recently evaluated by Krumholz & McKee. Finally, we discuss
the implications of these timescales for star formation efficiencies, the
disruption of gas by stellar feedback, mass segregation of stars, and the
longevity of turbulence in molecular clumps.
