  Assuming that the vacuum energy-momentum tensor is not exceptionally large,
we consider 4D evaporating black holes with spherical symmetry and evaluate the
proper distance $\Delta L$ between the time-like apparent horizon and the
surface of the collapsing matter after it has entered the apparent horizon. We
show that $\Delta L$ can never be larger than $\mathcal{O}(n^{3/2}\ell_p)$ when
the black hole is evaporated to $1/n$ of its initial mass, as long as $n \ll
a^{2/3}/\ell_p^{2/3}$ (where $a$ is the Schwarzschild radius and $\ell_p$ is
the Planck length). For example, the distance between the matter and the
apparent horizon must be Planckian at the Page time.
