The safe minimum dose for exposure with absolutely guaranteed non-infection is zero virions and the theoretical maximum duration a virion can stay infective is

The safe minimum dose for exposure with absolutely guaranteed non-infection is zero virions and the theoretical maximum duration a virion can stay infective is basically infinite, just with an infinitesimally small chance. It's more like a half-life than like a TTL counter. It's not impossible to get infected from someone a hundred miles away, just very, very unlikely. Early variations were sufficiently unlikely to successfully take root in a new host from small exposures to make it quite easy to push that R value below one, but omicron pushes the threshold for "good enough" far out.