... things fall into the realm of legend? In my story, the humans and non humans have been away from contact with Earth for enough time for their origin to have faded into the past. Gone, save for the odd little fairy tale and near derelict remnants of religions. But how much time must really pass for this to be? I have it brewing in my mind that it all depends on many, many factors, things that have happened since they left Earth. Was there a dark ages that obscured the history? Were the original settlers religious zealots who chose not to pass the information to the children? Did some kind of crisis happen which shifted focus away from maintaining a historical record. Many, many things. The reason it matters to me, and why I cannot just gloss it over as some uncountable number of years in the past is that I have reason to have some of the original technology still function to an extent. And I don't want to create "magical technology" that survives for millions of years and still powers on, a la the horrid remake of Planet of the Apes. I open the question to the floor for general discussion.
It depends entirely on the people. The Gaelic sennachies recorded thousands of years of history, and just when people began to admit that it was lost I and a few others found that it is still known in oral stories often dismissed as myths until compared with each other. It's not legend, it's historical fact. As said, it depends entirely on who we are dealing with, and the significance of the event.
Try taking a look at our own supposedly historical stories and go back in time until they start meeting your definition of legends. Certain aspects of the pilgrims coming to america, Columbus discovering that the world wasn't round. The stories of King Arthur is probably where I would say they start becoming legend and those are supposed to have taken place in the 6th century, he came to popularity during the 12th century due to the Historia Regum Britanniae, so you've got a rough estimate of 600ish years. The problem is that the higher the technology and education level, the longer period of time it would take to happen.
As long as it takes for reliable records to disappear, and accounts by witnesses are in limited supply. Legends require some uncertainty and some exaggeration. The secrecy that surrounds covert operations could make an operative a legend in only a few years. More public figures and events may require centuries, even millenia, to pass into legend (consider an entire civilization's influence).
It actually doesn't need to take very long. It could take just days, even. Think about it - when a famous person dies, rumors can easily start circulating about them and their lives, and since the person is no longer alive to claim the truth, who knows what could have happened? There's so many factors about this, and it's important that the focus be on these factors, and not when. If there are extremely reliable records that are easily accessible to a good part of the population, then perhaps it will take a long time before things become legends. But if there are no records at all, say, like as if everything was passed down orally, then it is extremely easy even within a couple of generations for people to believe historical things as legends.
