I don't think we can know that, but my guess is decades at least. It seemed like it had been there for a long time, what with the deserted Earthbound Village and all. Also the Blackbird had already been constructed, the Skyways were fully functional, and let's not forget about that Elemental Weapon place or whatever. It seemed like it had been there for a long time as well, and it was also known that they used the power of the Sunstone before the Mammon Machine came or something.
I assume it's been there for at least 100 years.
But something that seems strange to me is, why do they count down in years? The prehistoric age is in the past, but that's in the year 65,000,000 B.C. or w/e and Zeal is in 12,000 B.C., so it seems weird that they would count down instead of up. At some point the kingdom of Guardia was founded and they started at the year 0 again, and from that point they do count up.
Ah well, lots of weird stuff left to figure out.