  The tidal disruption event (TDE) AT2022cmc represents the fourth known
example of a relativistic jet produced by the tidal disruption of a stray star
providing a unique probe of the formation and evolution of relativistic jets in
otherwise dormant supermassive black holes (SMBHs). Here we present deep,
late-time Chandra observations of AT2022cmc extending to $t_{\rm obs} \approx
400$ days after disruption. Our observations reveal a sudden decrease in the
X-ray brightness by a factor of $\gtrsim 14$ over a factor of $\approx 2.3$ in
time, and a deviation from the earlier power-law decline with a steepening
$\alpha \gtrsim 3.2$ ($F_X \propto t^{-\alpha}$), steeper than expected for a
jet break, and pointing to the cessation of jet activity at $t_{\rm obs}
\approx 215$ days. Such a transition has been observed in two previous TDEs
(Swift J1644+57 and Swift J2058+05). From the X-ray luminosity and the
timescale of jet shutoff, we parameterize the mass of the SMBH in terms of
unknown jet efficiency and accreted mass fraction parameters. Motivated by the
disk-jet connection in AGN, we favor black hole masses $\lesssim 10^5 \ \rm
M_{\odot}$ (where the jet and disk luminosities are comparable), and disfavor
larger black holes (in which extremely powerful jets are required to outshine
their accretion disks). We additionally estimate a total accreted mass of
$\approx 0.1 \rm \ M_{\odot}$. Applying the same formalism to Swift J1644+57
and Swift J2058+05, we favor comparable black hole masses for these TDEs of
$\lesssim$ a few $\times 10^5 \ \rm M_{\odot}$, and suggest that jetted TDEs
may preferentially form from lower mass black holes when compared to
non-relativistic events, owing to generally lower jet and higher disk
efficiencies at higher black hole masses.
