It always struck me that the Communist regime lasted about 80 years - one person's long lifetime. As did the theocracy in Heinlein's Future History chart (2012-2100. Beginning with a vulgar, low-class, nearly illiterate televangelist with Old Money backing running for President and winning. Overthrown by a fed-up populace, complete with a harem revolt 80 years later.) First, Heinlein was a fairly decent observer of human nature on the ground in his day; second, 70-80 years feels to me - yeah - intuitive reasoning - like just about the right length for a basically unsustainable regime.

Nazism, of course less than 20 years (sound track from the finale to Wagner's magnum opus.) The Pathet Lao self-destructed spectacularly, as did the regime in Rumania whose pro-natalism policy filled the orphanages with starved and neglected children.

These fly-by-night dictatorships - Plutonian? Or something else?

Alabaster Decrepit Peacock