So how much time is needed exactly? Legion and the feltotem incident occurred in year 32 (dark mirror short story occurred shortly before the Legion demon invasion in year 32):
Shadowlands started in year 35, and Dragonflight starts in year 40 (I'm in favor of the timeskip - #47 by Erudician-1641), already 8 years after the events in Legion.
And seeing as how the entire history of events after the orc invasion of Azeroth to Legion occured in only 32 years. Hell according to the same blue post, BC was in year 26, and Shadowlands was in year 35, so everything BC on occurred in only 9 years.