Past Tense

Before King Vaas... Before Loe Kanum... Before Morrigan... Before Banshii... The lands before the continents knew of one another.
Only just the start of the tentative first meetings of those who will create the world of Meddus as it is now.

Warning: Will designate where which races originally hail from. So your 'Dust' bug must be located in these areas until I allow the sailing of ships.
Old Bloods are declining, Feral Bloods are the majority, but New Bloods are gaining numbers.

How much earlier in time than Ash, Mirror or Meddus is it playing?

Ash is when King Vaas was young. But Dust is before any of the continents sent diplomatic ships to eachother. The only ones you'll find sea fairing are Aceans or Water insects and whatever meddians they drag with them.
So I would say... atleast a few hundred years if not a few thousand.