The THz-field-driven streak camera has proven to be a powerful diagnostic-techniquethat enables the shot-to-shot characterization of the duration and the arrival time jitter offree electron laser (FEL) pulses. Here we investigate the performance of three computationalapproaches capable to determine the duration of FEL pulses with complex temporal structures fromsingle-shot measurements of up to three simultaneously recorded spectra. We use numericallysimulated FEL pulses in order to validate the accuracy of the pulse length retrieval in average aswell as in a single-shot mode. We discuss requirements for the THz field strength in order toachieve reliable results and compare our numerical study with the analysis of experimental datathat were obtained at the FEL in Hamburg - FLASH.