Question

How often is the Devovorga's Essence mini world change active?

I don't check world board much, but I want to kill the Devovorga bosses to get the achievements.

Answer 1

Old statistics point this mini world change (MWC) has (or had) ~1.5% chance of being triggered, which means once in ~67 days.

After the implementation of new set of MWCs with Update 10.80, there hasn't been gathered much data about triggered MWCs, so we can't be sure if the numbers changed. However, this is surely among the rarest MWCs.