The Power Density Spectrum (PDS) is one of the main tools used to characterise time variability. The regular monitoring of a large number of blazars with Fermi/LAT allows us to make a systematic study of the PDS at gamma-rays for a large sample of sources. In this presentation we describe the results of such an analysis, in particular a characterization of the PDS as a function of blazar class and other source parameters. We also discuss the evidence for characteristic time scales and time evolution of the PDS.