Microsoft has detailed the official support cycles for both Windows Phone 7.8 and Windows Phone 8 on its support page for the platform and posted in the table below:
As can be seen in the table, both Windows Phone 7.8 and 8 will be supported from the start dates until the respective end dates on September 9th and July 8th, 2014 respectively, during which time Microsoft will continue to offer security and other relevant updates, with the major caveat posted below and seen in the “Notes” table:
Microsoft 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. Update availability will also vary by country, region, and hardware capabilities.
While this means that support for Windows Phone 7 will officially end next September after three years and ten months on the market, any updates will be at the whim of local carriers, specific hardware and regional support, greatly reducing the possibility for consistent and uniform updates from hardware manufacturers and carriers.
As for the relatively short support cycle for Windows Phone 8 from its official release last December to next July, Microsoft has already confirmed that all current Windows Phone 8 devices will be able to be updated to the forthcoming major update from Windows Phone 8, expected in the fourth quarter of the year along with a new generation of hardware. The 18 month support timeline may also reflect Microsoft’s latest internal move to yearly updates for all Windows 8 platforms going forward in a bid to react faster to marketplace demand.
In terms of support, 18 months for the current Windows Phone versions may not seem like a long time, but Microsoft is likely taking into account rolling agreements which allow for early upgrades as soon as 18 months before completion, making short-term support more cost effective than maintaining support over the complete 2 year agreement cycle.
By doing so, the company is likely betting on owners continually upgrading instead of keeping older hardware for as long as possible in order to continue receiving the latest updates, though it still has to deal with carriers and their own testing and cycles for updates, making the aforementioned support cycles more of a best case scenario and less of a defined timetable, at least in the US where carriers still determine what updates get rolled out. The support cycles also do not take into account directly purchased unlocked hardware, which are more likely to be updated first ahead of carrier subsidized hardware.