On the official Windows Phone support site Microsoft has revealed when Windows Phone 8.1 will officially start being supported.
The life cycle for the OS will start on the 24th June and will last till the 11/7/2017.
Microsoft warns however:
Microsoft will make updates available for the Operating System, including security updates, for a minimum of 36 months after the lifecycle start date. These updates will be incremental, with each update built on the update that preceded it. Customers need to install each update in order to remain supported. The distribution of these incremental updates may be controlled by the mobile operator or the phone manufacturer from which you purchased your phone, and installation will require that your phone have any prior updates. Update availability will also vary by country, region, and hardware capabilities.
The first handset running the OS will however hit the market as early as next week, on the Nokia Lumia 630, but we expect numerous other handsets to be released soon after, and the official update to start rolling out on the 26th June or thereabouts.
See the page here.