As for when I would say picking a year is impossible. Beyond that I think a really good reference will be 3-5 years after you see that farm going driverless tractors. Then with that likely much of the spraying will be a driverless rotator with specific tasks done with drones.
As far as doing large scale commercial ag aviation, it's a novel idea that has many not so novel obstacles to begin to deal with in dealing with all that comes with that 3rd axis and close proximityto obstaclesor the groind. I dont think it will ever be less expensive per acre than any form of applications today.