Insider sources have divulged some impressive knowledge to the Petri IT Knowledgebase: Windows 10 has exceeded 100 million installs as of last week. This means it took Windows 10 just two months to do what Windows 8 did in eight months. That's the power of free (and good), we suppose.
All signs point to that number increasing even more before long: analysts claim more interest in the OS is just going up, Microsoft has recently signed a deal to get it over to China (which, if you live under a rock, is home to well over 1 billion potential customers), and the Surface Pro 4 is expected to be announced very soon.
Microsoft wants to hit 1 billion installs within three years. Given the rate of progress so far, that should be doable.