ABSTRACT We examine the long-term (rest-frame time-scales from a few months to ∼20 yr) X-ray variability of a sample of 2344 X-ray bright quasars from the Sloan Digital Sky Survey (SDSS) data release 14 quasar (DR14Q) catalogue, based on the data of the Spectrum-Roentgen-Gamma (SRG)/eROSITA All-Sky Survey complemented for ∼7 per cent of the sample by archival data from the XMM–Newton Serendipitous Source Catalogue. We characterize variability by a structure function, SF2(Δt). We confirm the previously known anticorrelation of the X-ray variability amplitude with luminosity. We also study the dependence of X-ray variability on black hole mass, MBH, and on an X-ray-based proxy of the Eddington ratio, λX. Less massive black holes prove to be more variable for given Eddington ratio and time-scale. X-ray variability also grows with decreasing Eddington ratio and becomes particularly strong at λX of less than a few per cent. We confirm that the X-ray variability amplitude increases with increasing time-scale. The SF2(Δt) dependence can be satisfactorily described by a power law, with the slope ranging from ∼0 to ∼0.4 for different (MBH, λX) subsamples (except for the subsample with the lowest black hole mass and Eddington ratio, where it is equal to 1.1 ± 0.4).