  Fermi's Golden Rule (FGR) applies in the limit where an initial quantum state
is weakly coupled to a {\it continuum} of other final states overlapping its
energy. Here we investigate what happens away from this limit, where the set of
final states is discrete, with a nonzero mean level spacing; this question
arises in a number of recently investigated many-body systems. For different
symmetry classes, we analytically and/or numerically calculate the universal
crossovers in the average decay of the initial state as the level spacing is
varied, with the Golden Rule emerging in the limit of a continuum. Among the
corrections to the exponential decay of the initial state given by FGR is the
appearance of the spectral form factor in the long-time regime for small but
nonzero level spacing.
