Announcing the official release date of Windows 10, Blogging Windows blog wrote that:
On July 29, you can get Windows 10 for PCs and tablets by taking advantage of the free upgrade offer, or on a new Windows 10 PC from your favorite retailer. If you purchase a new Windows 8.1 device between now and then, the Windows 10 upgrade will be available to you and many retail stores will upgrade your new device for you.
The initial release of Windows 10 on July 29, 2015 is for PC and tablets only. Windows 10 Mobile and editions for other forms of devices will come later.
With the release date available, it means that officially the free upgrade to Windows 10 offer will end on July 29, 2016. But once you have upgraded, Microsoft will continue to support it and keep it current for the lifetime of the device at no cost.
Microsoft has also started to activate the Get Windows 10 app where the Windows 7 and Windows 8.1 users can reserve free Windows 10 upgrade. The app also does the system compatibility check to ensure that the upgrade to Windows 10 is trouble-free.