It's a bit crass to talk about it, but it's an interesting thought experiment to imagine what the Nazis final death-count would have been if they had been left to run riot for 70'odd years like the Soviet "communists". Then there are the other fascistic governments of the 20th century to take into account. The Japanese weren't exactly benign overlords in China and Korea, then you have the central European nazi puppets happily marching their own populations off to the death camps.
Also worth remembering that 20-25 million or so of the "left" being decried here themselves died putting an end to far-right atrocity, and that's just the Soviets, it doesn't included untold numbers of Chinese who died fighting the Japanese invaders.
You are correct though. Totally different timescales involved, both utterly repugnant in their own ways, so it's a bit bizarre to try and turn it into a "who is worse" Top Trumps because there is nothing to suggest that if the other "side" had prevailed fewer people would have ultimately died. IMO the reality would have been an order of magnitude worse, because as much as the Soviets mistreated the various peoples they incorporated, they never had designs on total global domination and subjugation in the same way the Fascists did. Not quite so hell-bent on racial purity and ethnic genocide either.
History has proven that the far left are much more dangerous than the far right when given absolute power
I'd say this is highly contentious and debateable, because far-right regimes tend to flame-out far more quickly than the far left on the whole, yet they still put up pretty astonishing death-counts despite being comparatively short-lived.