Understanding the fate of semi-classical black hole solutions at very
late times is one of the most important open questions in quantum
gravity. In this talk, we provide a path integral definition of the
volume of the black hole interior and study it at arbitrarily late times
for black holes in various models of two-dimensional gravity. Because of
a novel universal cancellation between the contributions of the
semi-classical black hole spectrum and some of its non-perturbative
corrections, we find that, after a linear growth at early times, the
length of the interior saturates at a time, and towards a value, that is
exponentially large in the entropy of the black hole. This provides a
non-perturbative confirmation of the complexity equals volume proposal
since complexity is also expected to plateau at the same value and at
the same time.