From Waymo's 4 million mile announcement[0], they did their most recent million miles in 6 months (May 2017 to Nov 2017). The announcement also has the timeline of when they hit each million mile mark.
Also, I'm not sure this stat says anything besides how many cars they have on the road. Disengagement rates are probably still the most useful to gauge how good any company is doing.
Disengagement rates out of context don't indicate much. Going 10,000 miles up and down a lonely stretch of highway without an unplanned disengagement makes for a pretty number, but is worth less than going a mile in dense urban traffic with cyclists, pedestrians, left turns through busy uncontrolled intersections, double parked vehicles etc.
Also, I'm not sure this stat says anything besides how many cars they have on the road. Disengagement rates are probably still the most useful to gauge how good any company is doing.
[0] https://medium.com/waymo/waymos-fleet-reaches-4-million-self...