Windows 10 seems to be already active on more than 350 million devices around the world. And several oh them them are from the free upgrade from Windows 7 or Windows 8.1 products. However, the Windows 10 free upgrade offer ends July 29th, 2016.
So, if you have a qualified product for the upgrade, you have only few hours to make a free upgrade.
Also note that on early August there will be a big update package (the Anniversary Update) with a lot of new features, but you must have upgraded to Windows 10 before, in order to get this update.