Nowadays, polycrystalline Pb(Zr, Ti)O 3 (PZT) thin films are widely used in various applications such as piezoelectric actuators, sensors and Ferroelectric Random Access Memory. In this later case an important issue is the switching speed capability at low voltage of the ferroelectric domains as well as their size. The switching speed control of nano-sized domain is mandatory for the application of ferroelectric nanostructures in new more efficient devices. In the last years, many studies have been done on the switching dynamics of epitaxial PZT ferroelectric thin films via Piezoresponse Force Microscopy (PFM). PFM is a powerful technique to study ferroelectric materials at the nanoscale level as the domain structures can be controlled in a non-destructive way and the orientation of the polarization can be studied locally. We report here the switching dynamic study in polycrystalline PZT thin films with two different thicknesses on a large time scale (from 1 s down to 10-9 s). Both films present a similar grain size (R mean ≈45 nm) and allowed the observations of two different regimes for the polarization switching speed. We could evidence a tight connection of the nucleation and growth of nano-domains as well as of their success rate with the film thickness and grain structure.