There used to be this great stat site (see
http://redditmetrics.com/r/bitcoin -> tab Total Subscribers) that showed the total number of cumulative subscribers over timer, and a few other things. Unfortunately, the site stopped adding data in March 2018, although the historical data is still viewable there.
If the information provided by that site is correct, /r/Bitcoin had (dd/mm/yyyy):
02/01/2013 -> 13.239 subscribers
02/01/2014 -> 91.604 subscribers
02/01/2015 -> 147.073 subscribers
02/01/2016 -> 171.297 subscribers
01/01/2017 -> 198.612 subscribers
01/01/2018 -> 619.161 subscribers
04/03/2018 -> 759.794 subscribers
And now we know that 02/12/2018 -> 1.000.000 subscribers
The progression was very noticeable between 2017 and 2018, but largely concentrated on November 2017 onwards.