Random Headcanon: Lando Calrissian made the rank of General about three weeks after officially joining the Rebel Alliance. The on-paper justification for his extraordinarily rapid ascent is a tale of stirring heroism; the real reason involved the Rebel leadership taking one look at him and going “holy shit, this guy has actual administrative experience – put him in charge immediately”.
(That this would entail Calrissian being obliged to attend all of the same meetings as Han Solo, a man who made General by flying really fast, was not a factor in this decision, though it ultimately proved to be an entertaining bonus.)