I Know Nothing About Cloning Dinosaurs

I'm watching supernatural with my friend and she just paused and asked me:

How long do y'all think it took for people to forget mammoths? One generation, two, three? They got rarer and rarer, until the clan felled the last one that they would ever kill, and the hunters who were there would, for the rest of their lives, keep telling the story of how they once slayed the most elusive grand beast, that was only seen once a generation. And the youths would listen their descriptions of them, and though the description didn't make much sense - there was nothing else quite alike a mammoth that it could be compared to - they listened and thought that one day, they would encounter a mammoth, too.