In this scenario, my protagonist species of aliens stumble upon some pre-industrial civilization. While trying to establish contact with them to uplift their technology and science, they mess up badly and cause shock and panic among the population of the planet, which further devolves into hysteria, breakdown of the government, mass famine, war, and spike in criminal activity nearly on the entire planet. It's an absolute shitshow. In attempts to fix the mess, the aliens end up basically taking over the planet and establishing an occupation, now directly controlling all the major powers. They didn't want this outcome, but it is what it is now.
I'm making an assumption that the spacers culture will be sort of "virulent" and invasive, partially due to its novelty and exoticism from the point of view of the locals (they weren't particularly nationalist or conservative to begin with), and partially because of the overwhelming industrial and technological superiority of the aliens coupled with the fact that they're kind of in charge here and want to proceed with their plans of pulling the locals up the technological ladder. They're mostly hailed as saviors, at least in the initial years of the occupation, so there's no push back against their culture or presence among the native population as well.
Given these conditions, how many years - or, rather, generations, more accurately? - can it take for the original culture to dissolve and wither away under the influence of the more robust and invasive culture of an interstellar civilization, leading to the loss of a significant portion of the local lore?
By that, I mean that more than 70% of the population ceases practicing their original customs, adhering to their original religious beliefs, and so forth - the records about all of this might remain but they become historical and outdated.
Another thing to note is that the aliens don't actively try to replace the local culture, it happens on its own and almost by accident, with the whole thing going largely unnoticed.