Self-driving cars are not the only thing that Google has been working on automation in transport. Project Wing of Google Inc. experiments the same thing with air vehicles. This project has been around for more than two years now. Last year, a testing on single-wing drone has been done in Australia. Though it passed the test of delivering packages to farmers, the design was scrapped. But that didn't stop the experimental cargo-drone team from working furthur.
Recently Dave Vos, head of Google's Project Wing, stated that they want to go beyond drones. Their vision is to automate planes (even commercial passenger planes).
So if the self-driven cars hit the road and become success, experts say we can expect self-driven flights in next 6 to 8 years.
Though experts believe that self-driven flights will be available within the above mentioned time window, I guess it will take some more time than that.
The self-driven cars of Google are not into market completely. Currently only four states in U.S allow driver-less cars to enter their streets for testing. Google haven't released the fully functioning car yet. So it will take some time for self-driven cars to root well among us. If things go well with cars, then flight idea will be easy for them. If not, then they will have a hard time convincing people about autonomous products.
Moreover the risk is greater in space than in roads. Road is just two dimensional. It's been almost 7 years now (self-driving car project started in 2008) and Google couldn't come up with the product yet. This explains the complexity in work. Now comes space, which is three dimensional. So it will take more time to deal with the increased complexity.
According to me self-driven flights will be my kids' transport medium. The next generation will use this tech, if things go good :)