  The physical scenario responsible for gamma-ray flaring activity and its
location for Flat Spectrum Radio Quasars is still debated. The study of the
statistical distribution of waiting-times between flares (the time intervals
between consecutive activity peaks) can give information on the distribution of
flaring times, and constrain the physical mechanism responsible for gamma-ray
emission. We adopt here a Scan-Statistic driven clustering method (iSRS) to
recognize flaring states within the FERMI-LAT data, and identify the time of
activity-peaks. Results: Flares waiting times can be described with a
poissonian process, consisting of a set of overlapping bursts of flares, with
an average burst duration of about 0.6 year, and average rate of 1.3/y . For
waiting times below 1d host-frame we found a statistically-relevant second
population, the fast-component, mainly from CTA 102 data. The period of
conspicuous detection of the fast component for CTA 102 coincides with the
crossing-time of the superluminal K1 feature with the C1 stationary feature in
radio reported in Jorstad et al. (2017); Casadio et al. (2019). Conclusions: To
reconcile the mechanism proposed in Jorstad et al. (2017); Casadio et al.
(2019) with the bursting-activity, we have to assume that plasma streams with a
typical length of about 2pc (in the stream reference-frame) reach the
recollimation-shock. Otherwise, the distribution of waiting-times can be
interpreted as originating from relativistic-plasma moving along the jet for a
deprojected length of about 30-50pc (assuming a Lorentz-factor=10), that
sporadically produces gamma-ray flares. In magnetic-reconnection scenario,
reconnection events or plasma injection to the reconnection-sites should be
intermittent. Individual plasmoids can be resolved in a few favourable cases
only (Christie et al., 2019); they could be responsible for the fast component.
