The PIP-II Injector Test (PIP2IT) is being built at Fermilab. As of now, a 15-mA DC, 30-keV H- ion source, a 2 m-long Low Energy Beam Transport (LEBT), a 2.1- MeV CW RFQ, followed by a 10-m Medium Energy Beam Transport (MEBT) have been assembled and tested. A Fast Faraday Cup (FFC) installed in the MEBT measures the length of a beamlet cut out of the bunch by a small-size entrance hole of the FFC. The information about the bunch length measured at various settings allows for reconstruction of the longitudinal beam dynamics and optimization of injection into the first cryomodule. These measurements are compared with simulations by the beam dynamics codes TRACEWIN and TRACK. The report describes the experimental procedures of the bunch length measurements with the FFC, presents the measurement results, and compares them with simulations. One of important experimental observations, confirmed by simulations, is the dependence of the FFC beamlet length on the radial position across the beam.