We use the quantum unimodular theory of gravity to relate the value of the
cosmological constant, $\mathrm{\Lambda}$ , and the energy scale for the emergence of
cosmological classicality. The fact that $\mathrm{\Lambda}$ and unimodular time are
complementary quantum variables implies a perennially quantum Universe should
$\mathrm{\Lambda}$ be zero (or, indeed, fixed at any value). Likewise, the smallness of
$\mathrm{\Lambda}$ puts an upper bound on its uncertainty, and so a lower bound on the
unimodular clock's uncertainty or the cosmic time for the emergence of
classicality. Far from being the Planck scale, classicality arises at around $7\times {10}^{11}$ GeV for the observed $\mathrm{\Lambda}$ , and taking the region of
classicality to be our Hubble volume. We confirm this argument with a direct
evaluation of the wavefunction of the Universe in the connection representation
for unimodular theory. Our argument is robust, with the only leeway being in
the comoving volume of our cosmological classical patch, which should be bigger
than that of the observed last scattering surface. Should it be taken to be the
whole of a closed Universe, then the constraint depends weakly on ${\mathrm{\Omega}}_{k}$ :
for $-{\mathrm{\Omega}}_{k}<{10}^{-3}$ classicality is reached at $>4\times {10}^{12}$ GeV.
If it is infinite, then this energy scale is infinite, and the Universe is
always classical within the minisuperspace approximation. It is a remarkable
coincidence that the only way to render the Universe classical just below the
Planck scale is to define the size of the classical patch as the scale of
non-linearity for a red spectrum with the observed spectral index ${n}_{s}=0.967(4)$ (about ${10}^{11}$ times the size of the current Hubble volume). In the
context holographic cosmology, we may interpret this size as the scale of
confinement in the dual 3D quantum field theory, which may be probed (directly
or indirectly) with future cosmological surveys.