Today Microsoft announced that the official support for Windows Phone 8.1 will kick off on June 24th 2014. This means that the official rollout should kickoff the official rollout of the massive system update (alongside the Lumia 930 launch perhaps?).
The WP 8.1 update will get the usual guaranteed 36 months of software support (which refreshes with every major rollout).
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.