  We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100
MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show
high activity in this period of time. We characterise the duty cycle of the
source by comparing the time spent by the sources at different flux levels. We
consider in detail the light curves covering periods of extreme flux. The large
number of high-energy photons collected by LAT in these events allows us to
find evidence of variability on timescales of few hours. We discuss the
implications of significant variability on such short timescales, that
challenge the scenario recently advanced in which the bulk of the gamma-ray
luminosity is produced in regions of the jet at large distances (tens of
parsec) from the black hole.
