Well I looked it up, have not looked at any info in several years, seems like a certain percentage will fly up to the 12 miles.
Aka it's either already there, or will be shortly. All they have to do is find one tree in between a said area of uninvested ash and their current territory, and it will spread. Other articles said they can go further, and even then, if it's breezy, I can see them going a heck of a lot further than 10-15 miles.
We dont have any large areas of ash here, its all towns and farms spread out, i think it has slowed the spread here.