In the present study, the P-wave peak displacement (Pd) is evaluated to rapidly estimate the magnitude and duration of the earthquake source time function, for application to Earthquake Early Warning (EEW) systems. To this regard, we average the logarithm of the normalized Pd by hypocentral distance over all available stations at any time windows (LPDT), starting from the P-phase onset. Mathematically, the LPDT curve increases with time until it reaches a constant value (plateau), which is related to the magnitude of the earthquake. From the obtained observations, the corner time of the LPDT at the beginning of the plateau can be considered as a proxy of the source duration/length of the earthquake. This approach has been applied and tested using the recent earthquakes occurred in central Italy, with magnitude range 3.5-6.5.