  We report a large set of experimental data which demonstrates that a simple
yield stress fluid, i.e. which does not present aging or thixotropy, exhibits
transient shear banding before reaching a steady state characterized by a
homogeneous, linear velocity profile. The duration of the transient regime
decreases as a power law with the applied shear rate $\dot\gamma$. This power
law behavior, observed here in carbopol dispersions, does not depend on the gap
width and on the boundary conditions for a given sample preparation. For
$\dot\gamma\lesssim 0.1$ s$^{-1}$, heterogeneous flows could be observed for as
long as 10$^5$ s. These local dynamics account for the ultraslow stress
relaxation observed at low shear rates.
