The time dependence of the parameter of the Gutenberg-Richter (GR) magnitude distributions is computed for correlated foreshock sequences of earthquakes, by using the geometric-growth model of earthquake focus, the magnitude distribution of correlated earthquakes and the time-magnitude correlations, derived recently. It is shown that this parameter decreases in time in the foreshock sequence, from the background values down to the main shock. If correlations are present, this time dependence and the time-magnitude correlations provide a tool of monitoring the foreshock seismic activity. We discuss a possibility of estimating the moment of occurrence of a main shock by such an analysis of a foreshock sequence. The discussion is applied to two Vrancea main shocks. The appreciable limitations of such an estimation are discussed.