It was that long.
Essentially, Auroc finds a planet with primitive Threshlings on it. Nothing overly special except their exceptional adaptability.
Shortly after meeting the First Race, a small splinter group settles on Reshlan as well and Auroc starts to...tinker.
He discovers a means to slowly alter things and gets the two races to eventually breed into what we would recognize as the current Threshling species.
He pops in as a mythological figure a lot over the course of thousands of years, nudging things here and there, performing ‘miracles’ and adding more of his ‘taint’ to things, building up their belief system in him as some sort of ‘God’. He can’t do large scale Taint yet, he doesn’t have the magic for that, but he is building up to that as he further breeds the Threshlings into a highly magical race. He mainly contacts the royal family when he does take a more personal hand, letting them spread his edicts as they acted as the head of his ‘church’ essentially.
While doing this he is also exploring the universe(a), timelines, and pocket dimensions as well, which eats up a lot of time.
Eventually, he starts staying on Reshlan more and more as the magic starts being more effective for his uses. The current Priestess class is established and he starts working with several of their higher-ups in secret (eventually the Maven being one such).
So yes, he’s influenced them for their entire history. He just didn’t make his home there and presence fully known to the entire population until probably close to the last 50 to 200 years.