EDIT: Citations
"Google Self-Driving Car Project Monthly Report | December 2015" https://static.googleusercontent.com/media/www.google.com/en...
"We’re currently averaging 10,000-15,000 autonomous miles per week on public streets"
There are ~100K Model S vehicles. Somewhere around VIN 55K is when autopilot hardware was introduced (http://www.teslamotorsclub.com/showthread.php/44787-Autopilo...). Let's conservatively say that there are 40K Model S vehicles out there with self-driving hardware that are refining their data, and consequently, algorithms Tesla is updating. 40K vehicles * 12K miles/year/vehicle (average) = 40,000,000 miles/month (!!!) in "autonomous experience" Tesla is gaining. (Fun fact: The average human drives ~500K miles in their lifetime)
"According to Fred Lambert from Electrek, Musk referred to the first round of Model S owners with Autopilot as "expert trainers" that will collect and deliver tons of important data for Tesla's intelligence network. Musk said the system should improve each week as the driving algorithm constantly updates, adding Model S drivers could add around 1 million miles of new data every day."
http://www.techinsider.io/elon-musk-explains-tesla-autopilot...
That's a lot of progress being made each month.