We present THz photoconductivity measurements on Corbino-shaped GaAs/AlGaAs heterostructures. The THz source is a pulsed p-Ge laser, which provides photon frequencies of 1.7 THz to 2.5 THz (corresponding to wavelengths of 180–120 μm). We investigate the relaxation process from the dissipative state to the quantum Hall state time-resolved and find that the relaxation time depends on the applied voltage and on the mobility of the sample. Relaxation times of approximately 10 ns to over 200 ns are observed. A simple picture is suggested to explain the results. In addition, spectrally resolved measurements are discussed. The short response time and the useful spectral selectivity together with the high sensitivity make QH devices promising for high-performance THz detectors.