I did a linear regression on the data provided and I estimate that it will reach 2 billion downloads in 18 months, which is January 2017. Also, according to my model, the total downloads at the end of this year will be 1,264,470,661. Here is my statistical analysis: https://www.learningpenguin.net/myblog/2015/08/11/estimate-of-time-for-wordpress-plugin-directory-surpassing-two-billion-downloads/

Time will tell if my model is accurate or not. :-)