I don't think it's gonna' be chiseled in stone anywhere. For lack of anything better, what about comparing readings to that of ten or a hundred feet of 6AWG? There may be some numbers in IEEE 837, but that's for substation ground connectors. It may be reasonable to take readings now, and check them again in a year.
IIRC, the NEC used to suggest "acceptable" megger readings, but later they were tossed out because some interpreted them as mandatory, and that wasn't the intent of the code panel. Within obvious limitations, changes in readings may be more revealing of potential degradation than a one-time reading without any baseline data.
Many agree that the nominal three-point 25-ohm ground resistance is OK in a many cases; in other situations that reading would be criminal.
If you are used to seeing ductor readings, you are probably are more qualified to evaluate numbers than many others.