20th century data have been used to determine repeat times, T, of main shocks in seven of the most active seismogenic sources of shallow earthquakes in Greece. It is observed that T depends on magnitude, Mp, of the preceding main shock and not on the magnitude, Mf, of the following main shock. This means that the time-predictable model and not the slip-predictable model is valid for the main shocks of these sources. A relation of the form log T = cMp + a fits the data. While the parameter “a” depends on the seismicity of each source and on the minimum (cutoff) magnitude of the main shocks considered, a representative of all sources value equal to 0.3 is found for parameter c when Mp is surface-wave magnitude and T is measured in yr.