The Microsoft Product Lifecycle support page indicates Windows Phone 8 and 7.8 are already past their lifecyle start dates. According to the listing, Microsoft plans to end mainstream support for Windows Phone 8 in July 2014, just 16 months from now, and support for Windows Phone 7.8 will end later in September 2014.
Regarding the schedule, Microsoft said it “will make updates available for the operating system on your phone, including security updates, for a period of 18 months after the lifecycle start date. Distribution of the updates may be controlled by the mobile operator or the phone manufacturer from which you purchased your phone.” Microsoft hasn't made any commitment to updates for every compatible device.
- PCWorld