Windows 8.1 Support Lifecycle Policy

Windows 8 customers will have two years to move to Windows 8.1 after the General Availability of the Windows 8.1 update to continue to remain supported under the Windows 8 lifecycle. 

http://support.microsoft.com/gp/msl-Windows-81

http://www.neowin.net/news/windows-8-users-have-two-years-to-update-to-windows-81-before-support-ends