  Effective field theories break down inside large black holes on macroscopic
scales when tidal forces are string-sized. If $r_0$ is the horizon radius and
$\alpha'$ is the square of the string scale, the 4D Schwarzschild interior is
strongly curved at $ \big(r_0 \alpha' \big)^{1/3}$. Infalling massless probes
that reach this scale stretch and become excited strings. I generalize this
picture for a wide class of black hole solutions in string theory. For the
black hole dual to the large-$N$ BFSS model in a thermal state, and denoting
$\ell_P$ the Planck length, tidal forces are stringy at $r_0
\left(\frac{r_0}{N^{1/3} \ell_P}\right)^{3/11}$, which is greater than the
scale where string perturbation theory breaks down for sufficiently large
$r_0/\ell_P$. For 4D Kerr, there is a range of spin parameters for which the
inner horizon is to the future of the scale of stringy curvature. These results
specify the portion of black hole interior solutions where effective field
theory can be used; beyond these scales, one must resort to other methods.
