  If very frequent periodic measurements ascertain whether a quantum system is
still in its initial state, its evolution is hindered. This peculiar phenomenon
is called quantum Zeno effect. We investigate the large-time limit of the
survival probability as the total observation time scales as a power of the
measurement frequency, $t\propto N^\alpha$. The limit survival probability
exhibits a sudden jump from 1 to 0 at $\alpha=1/2$, the threshold between the
quantum Zeno effect and a diffusive behavior. Moreover, we show that for
$\alpha\geq 1$ the limit probability becomes sensitive to the spectral
properties of the initial state and to arithmetic properties of the measurement
periods.
