I wish Tesla would update those numbers more frequently, but it looks like they stopped doing that in 2016. However they did enough updates in 2016 that allow us to estimate a trend of something on the order of 1 million miles per day.
4/9/2016 - 47 [1]
5/24/2016 - 100 [2]
6/30/2016 - 130 [3]
10/7/2016 - 222 [4]
11/13/2016 - 300 [5]
This rate continuing linearly would put us at roughly 900 million today. A linear accumulation of miles ignores that Tesla has roughly twice the cars on the road that they had in 2016 when those numbers were published. I have no idea their current mileage total has reached, but I feel safe in saying it is over a billion miles.
The best source I've been able to find so far is a tweet by Elon Musk in 2016 saying "Cumulative Tesla Autopilot miles now at 222 million".
[1]https://twitter.com/elonmusk/status/784487348562198529?ref_s...