One of the most challenging tasks for extreme ultraviolet, soft and hard X-ray free- electron laser photon diagnostics is the precise determination of the photon pulse duration, which is typically in the sub 100 fs range. For some purposes like the FEL tuning, an estimate of the pulse duration is required in real time and with minimal experimen- tal effort. Therefore a method has been developed to evaluate the photon pulse spectra during run-time, yielding an estimate for the photon pulse duration. The developed method makes use of the statistical properties and spectral correlation...