It may not have taken them long, though it may have been a process they had to quickly had to adapt to once the separate herds heading towards the Great Valley started to combine, maybe to help fight off dangers like sharpteeth at first. Then some like Ducky's parents, and others, may have decided it is safer to remain in a large group, and things may have developed from there.
His mother's reaction seems less full speciest like Threehorn has and more like, it's always been this way. That may have been their feelings, though some like Ducky's parents & Petrie's mother didn't have these feelings at all. Petrie's mother I'm guessing based on how Petrie acts when he first meets Littlefoot & Ducky. He doesn't act like a speciest or that he should not be talking to non flyers, much like Ducky was. Unlike Littlefoot who had started to do the same sort of thing that his mother may have done, but since he was so young it was easy for him to unlearn it and put it aside. So maybe not as deeply ingrained as Threehorn and Cera's are. Though she's partially lost it, she still has some of it even seen in some of the tv series episodes.
