We present the analysis of the ensemble structure function of the soft X-ray variability on a sample of about 3000 quasars. Our data are based on the observations of the ROSAT and XMM-Newton satellites, cross-correlated with the Sloan Digital Sky Survey Quasar catalogue. We measure for the first time the structure function up to a rest-frame time interval as large as 20 years (the longest time interval investigated through this type of study). The results of our analysis show that the X-ray variability of quasars still increases at these long time lags. This will be discussed in relation to the physical size of the emitting region.