  The shear-induced fluidization of a carbopol microgel is investigated during
long start-up experiments using combined rheology and velocimetry in Couette
cells of varying gap widths and boundary conditions. As already described in
[Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that
the fluidization process of this simple yield stress fluid involves a transient
shear-banding regime whose duration $\tau_f$ decreases as a power law of the
applied shear rate $\gp$. Here we go one step further by an exhaustive
investigation of the influence of the shearing geometry through the gap width
$e$ and the boundary conditions. While slip conditions at the walls seem to
have a negligible influence on the fluidization time $\tau_f$, different
fluidization processes are observed depending on $\gp$ and $e$: the shear band
remains almost stationary for several hours at low shear rates or small gap
widths before strong fluctuations lead to a homogeneous flow, whereas at larger
values of $\gp$ or $e$, the transient shear band is seen to invade the whole
gap in a much smoother way. Still, the power-law behaviour appears as very
robust and hints to critical-like dynamics. To further discuss these results,
we propose (i) a qualitative scenario to explain the induction-like period that
precedes full fluidization and (ii) an analogy with critical phenomena that
naturally leads to the observed power laws if one assumes that the yield point
is the critical point of an underlying out-of-equilibrium phase transition.
