We present an ab initio calculation of the band-to-band internal-conversion rate of the _nuc eV isomeric transition in [229]ThO2. Because the nuclear transition energy exceeds the electronic band gap of ThO2, the isomer can decay nonradiatively by resonantly promoting a valence electron into the conduction band. We formulate this process as a Brillouin-zone sum over vertical interband transitions weighted by local Th-centered hyperfine matrix elements, which are evaluated directly from all-electron full-potential linearized augmented-plane-wave Bloch spinors. A finite nuclear magnetization model is included to regularize the short-range hyperfine interaction and to account for the Bohr-Weisskopf effect. After applying scissor shifts to span the experimentally reported ThO2 band gaps, we find calculated internal-conversion lifetimes in the range of 1 - 16 µs. The lifetime increases strongly as band gap approaches _nuc because the resonant interband phase space at the nuclear transition energy is reduced. For the larger reported ThO2 gaps, the calculated lifetime is comparable to the measured conversion-electron Mossbauer lifetime [Nature 648, 300 (2025)]. Our analysis implies that choosing solid-state hosts with bandgap values slightly lower than _nuc can optimize solid-state nuclear clock performance with internal conversion electron readout.