Observed cooling timescale for the electrons emitting at the break frequency ν_b as a function of the magnetic field and for different distances from the central engine. It is assumed that particles cool via synchrotron and self-Compton processes (considering, for the latter, only the fraction of the synchrotron spectrum below hν/(m_ec²) = 1/γ_b). The black dashed line indicates the synchrotron cooling timescale only. The blue line is the minimum variability timescale found by assuming that the Poynting flux remains constant beyond the acceleration phase, leading to B′ ∝ R⁻¹ (see Sect. 3). As reference, a redshift z = 1 has been considered. The orange horizontal line corresponds to a typical exposure time of 1 s.