The DS took 20 months to reach 10m units in Japan. Although it was pretty slow in the start it has climbed very quickly and is jumping up all the time.
The Wii currently sits at 2m units and IMO will be maybe around the 6m come the end of the year, so reckon it will be well into next year, probably closer to the end of the year before it will be close to 10m units in Japan alone.
Seeing as people are beginning to think that the Wii will have not much more than a 3 year lifespan, 10m units in Japan seems about right....but it could take off big style like the DS and end up selling 20m+...who knows....the sales are good so far.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)
Essay Pro