We calculate the hydrodynamic time scales for a spherical ultra-relativistic shell that is decelerated by the ISM and discuss the possible relation between these time scales and the observed temporal structure in -ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities and precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not ned

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... , astrophysical parameters. The dierence between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts' durations.