Prompt Emission Properties

Quantifying the duration of a GRB is a non-trivial process - the typical quoted value of t90 is a measure of the time during 5-95% of the background-subtracted photons arrive at the detector.  However, t90 depends on the sensitivity of the instrument (more sensitive instruments will have larger t90 values for a given GRB, due to the "tip of the iceberg" effect), the bandpass (GRBs typically have shorter durations at higher energies), and of course the source redshift (cosmological time dilation).  So it is worth taking any measure of GRB "duration" with a grain of salt.
Nonetheless, it became clear very early on \cite{Kouveliotou_1993} that the distribution of the duration of GRBs was bimodal (Figure 3).  For reasons discussed above, the exact dividing line depends on the properties of the instrument of interest.  But classically the class of short-duration GRBs has been defined as those having a t90 duration of < 2 s.