  A neutron-spin experimental test of the quantum Zeno effect (QZE) is
discussed from a practical point of view, when the nonideal efficiency of the
magnetic mirrors, used for filtering the spin state, is taken into account. In
the idealized case the number N of (ideal) mirrors can be indefinitely
increased, yielding an increasingly better QZE. By contrast, in a practical
situation with imperfect mirrors, there is an optimal number of mirrors, N_opt,
at which the QZE becomes maximum: more frequent measurements would deteriorate
the performance. However, a quantitative analysis shows that a good
experimental test of the QZE is still feasible. These conclusions are of
general validity: in a realistic experiment, the presence of losses and
imperfections leads to an optimal frequency N_opt, which is in general finite.
One should not increase N beyond N_opt. A convenient formula for N_opt, valid
in a broad framework, is derived as a function of the parameters characterizing
the experimental setup.
