  To extract the information that the Mg II NUV spectra (observed by the
Interface Region Imaging Spectrograph; IRIS), carries about the chromosphere
during solar flares, and to validate models of energy transport via model-data
comparison, forward modelling is required. The assumption of statistical
equilibrium is typically used to obtain the atomic level populations from
snapshots of flare atmospheres, due to computational necessity. However it is
possible that relying on statistical equilibrium could lead to spurious
results. We compare solving the atomic level populations via statistical
equilibrium versus a non-equilibrium time-dependent approach. This was achieved
using flare simulations from RADYN alongside the minority species version,
MS_RADYN, from which the time-dependent Mg II atomic level populations and
radiation transfer were computed in complete frequency redistribution. The
impacts on the emergent profiles, lightcurves, line ratios, and formation
heights are discussed. In summary we note that non-equilibrium effects during
flares are typically important only in the initial stages and for a short
period following the cessation of the energy injection. An analysis of the
timescales of ionisation equilibrium reveals that for most of the duration of
the flare, when the temperatures and densities are sufficiently enhanced, the
relaxation timescales are short ($\tau_{\mathrm{relax}}<0.1$ s), so that the
equilibrium solution is an adequate approximation. These effects vary with the
size of the flare, however. In weaker flares effects can be more pronounced. We
recommend that non-equilibrium effects be considered when possible, but that
statistical equilibrium is sufficient at most stages of the flare.
