  A theoretical framework is presented for the estimation of nucleation times
in systems with Brownian type dynamics. This framework is applied to a
prototype system: the two-dimensional Ising model with spin-flip dynamics in an
external magnetic field. Direct simulation results for the nucleation times,
spanning more than four orders of magnitude, are compared with theoretical
predictions. There are no free parameters; all system properties required by
the theory are determined by simulations in equilibrium systems. Theoretical
and simulation values are found to agree in most cases within 20%.
