While we're not yet in that kind of numbers, and 2018 is just around the corner, Windows 10 has passed an impressive milestone. Currently Windows 10 is used by over 600 million users. And you have to remember that the whole Windows 10 phone thing failed miserably which doesn't really help with the target.
Even if one billion might be a little bit off, Windows 10 is growing rather fast. In May Microsoft said that Windows 10 was in half a billion devices worldwide, which makes give us a 100 million user growth over six months. If the rate continues, and that is a big if, Microsoft would pass 1 billion mark around this time of the year in 2019.
That is not Microsoft's prediction, though, as the company has apparently decided to refrain from commenting on new predictions.