  X-ray variability in Active Galactic Nuclei (AGN) is commonly analysed in
terms of the Power Spectral Density (PSD). The break observed in the power
spectrum can be interpreted as a characteristic X-ray variability time scale.
Here we study variability properties within the framework of clumpy accretion
flows, in which shocks between accreting elements account for the UV and X-ray
emissions. We derive a characteristic X-ray time scale, $\tau_{X}$, and compare
it with the measured PSD break time scale, $T_{B}$. A quite good agreement is
found in both magnitude and trend. In particular, the model dependence on black
hole mass and accretion rate precisely reproduces the empirical relation
obtained by McHardy et al. (2006). We suggest a possible physical
interpretation of the break time scale and briefly discuss the related aspects
of optical/UV variability and correlations between different wavelengths.
