I think we can apply the Pareto principle here. The car seems to work surprisingly well in ideal conditions, but probably doesn't in less than ideal ones (let's see it operating in a snow storm!). So, let's say that 80% of the work is done, leaving 20% to fix all the little edge cases that are bound to pop in the real world.

According to Wikipedia, the first truly autonomous cars started to appear in the 1980s. Let us round that off to 30 years of development. According to the principle, that means 30 years represents 20% of the time.

Thus, 120 more years before they become available. If the ownership model persists, I'd add another 5 years to let people replace their existing vehicles to reach common status. If the shared fleet model takes hold, as many suggest it will, then that number may be reduced somewhat.