So. I was reading through the HoA annotations (chapter 63), and I found this quote:
So, if he was one of the first Allomancers, and only allomancers can detect the Well, what does that mean about the previous Hero of Ages? My theory? There wasn't one. I think that Preservation made his prophecies and entraped Ruin's mind exactly 2048 years before the WoA.
Where do I get the date?
"Wait" you say. "Couldn't Preservation have made someone a Mistborn like Sazed did to Spook?" Yes. That's possible (darn it....) "Or maybe the prophecies were good enough that people knew where it was, so when the time came, they sent someone in."
You know what random person? You're right. I think that it's very possible that there is a third generation, where the prophecies were effective enough to send someone in without giving up the Power as Vin did.
Except that three generations doesn't seem very special. And plus, it seems like Preservation's mind would've been alert enough to block Ruin from changing the prophecies for at least a little bit. So lets add a fourth generation.
That even makes it work better. It means there's 4 generations, which is a much better number than 2. Leras entraped Ruin and then the first two generation when they used the prophecies, and then there's the third generation of Alethi, when Preservation used mists, and Ruin countered with the Deepness, and the fourth generation of Vin and Sazed.
So, in other words, 4096 (or 4^6, or 16^3 or 2^12) years ago, Leras betrayed Ati, captured his mind in a well, and made it known to the Terris people the prophecies of the Hero of Ages. We have prophecies about that old here on this earth today. Hey, Jacob and Joseph are dated about that time period.
So, no, it's definitely not absolute. But I think that it makes sense. And I think that any other great numbers (like 65536) are too unrealistic.