  Theories of modified gravity, in both the linear and fully non-linear regime,
are often studied under the assumption that the evolution of the new (often
scalar) degree of freedom present in the theory is quasi-static. This
approximation significantly simplifies the study of the theory, and one often
has good reason to believe that it should hold. Nevertheless it is a crucial
assumption that should be explicitly checked whenever possible. In this paper
we do so for the Vainshtein mechanism. By solving for the full spatial and time
evolution of the Dvali-Gabadadze-Porrati and the Cubic Galileon model, in a
spherical symmetric spacetime, we are able to demonstrate that the Vainshtein
solution is a stable attractor and forms no matter what initial conditions we
take for the scalar field. Furthermore,the quasi-static approximation is also
found to be a very good approximation whenever it exists. For the best-fit
Cubic Galileon model, however, we find that for deep voids at late times, the
numerical solution blows up at the same time as the quasi-static solution
ceases to exist. We argue that this phenomenon is a true instability of the
model.
