This answer provided by someone who is in the know, but not on this forum.
The correct answer is when the FAA stop playing games and moving the goal posts on what they had been already prepared to sign off on. Recent article by Paul Bertorelli has details.
As for Australia, some time after the USA production gets to a point where TEL is no longer viable to produce (Innospec) and Viva can get their restructuring in place, or anyone decides to import from the USA.
As for Chieftains......they will LOVE it, as will any TC piston aircraft. Supper Connie could run full MAP again if they wanted to. G100UL perfamnce testing has shown it is more like a 100/160 fuel so better detonation margin in rich mixture states than the old purple avgas at a 145 rating. All piston aero engines will run cleaner and longer and hopefully down the track without TEL deposits to be handled by the oil, products like Mobil AV1 could make a comeback, thus having better oil and longer service intervals. Even if it is a few c/l dearer, there will be many cost benefits
Swift fuel is not the answer, not suitable across the fleet and not fully fungible. And expensive by comparison to produce.
Patience is required, as we should have had this on the market in the USA by 2017. Again refer the article mentioned above.