Within four years, Google hopes to have the autonomous vehicles commercially available and citizens throughout the U.S. regularly riding in them, Urmson said.

Google Inc. threw the auto industry another curve ball Wednesday, saying its autonomous cars should be on public roads within five years without having to drive through a thicket of regulatory red tape.

Both sources are from 2015, the first one from january, second from last week. I think it unlikely (<10%) that they are going to overperfom by more than a single year, and very likely (70%) to take at least a year longer.

Remember, its not only a technological problem, but the inertia in a vast system. Insurance+lawmakers etc need to finish their side as well.

Elon Musk, someone who can reasonably called domain expert on innovative car systems, gives the same range:

The three-year estimate, Musk says, is when he thinks the technology will be ready. It could take longer for government regulations around the issue to go into effect. He acknowledged that it will take time for regulators to say "it's okay to go to sleep in the car" -- maybe another one to three years.

Note that I only looked up Musks opinion after I wrote the first part.