  We have characterized the typical temporal behaviour of the bright X-ray
flares detected from the three Supergiant Fast X-ray Transients showing the
most extreme transient behaviour (XTEJ1739-302, IGRJ17544-2619,
SAXJ1818.6-1703). We focus here on the cumulative distributions of the
waiting-time (time interval between two consecutive X-ray flares), and the
duration of the hard X-ray activity (duration of the brightest phase of an SFXT
outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting
the cumulative distribution of waiting-times, it is possible to identify the
typical timescale that clearly separates different outbursts, each composed by
several single flares at ks timescale. This allowed us to measure the duration
of the brightest phase of the outbursts from these three targets, finding that
they show heavy-tailed cumulative distributions. We observe a correlation
between the total energy emitted during SFXT outbursts and the time interval
covered by the outbursts (defined as the elapsed time between the first and the
last flare belonging to the same outburst as observed by INTEGRAL). We show
that temporal properties of flares and outbursts of the sources, which share
common properties regardless different orbital parameters, can be interpreted
in the model of magnetized stellar winds with fractal structure from the
OB-supergiant stars.
