  We study the stochastic coagulation equation using simplified models and
efficient Monte Carlo simulations. It is known that (i) runaway growth occurs
if the two-body coalescence kernel rises faster than linearly in the mass of
the heavier particle; and (ii) for such kernels, runaway is instantaneous in
the limit that the number of particles tends to infinity at fixed collision
time per particle. Superlinear kernels arise in astrophysical systems where
gravitational focusing is important, such as the coalescence of planetesimals
to form planets or of stars to form supermassive black holes. We find that the
time required for runaway decreases as a power of the logarithm of the the
initial number of particles. Astrophysical implications are briefly discussed.
