Plaffo.com reports on the life cycle support dates of Windows Phone 7.8 and Windows Phone 8.
According to the table Windows Phone 7.8 will receive security updates until 9 September 2014, while Windows Phone 8 will sunset on the 8th July 2014.
The date relates to how long Microsoft will make updates, including security updates available for the operating system, and it is of note that the date is not from when the phone is sold but when the cycle started.
This means for example a Windows Phone 8 handset sold in July 2013 will have only one year of guaranteed support.
In addition Microsoft notes:
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.
On the other hand the OS support date is reset with any never version of the OS, so a Windows Phone 8.5 or 9 update in November 2013 would bring along its own 18 month of security updates. Microsoft has already promised all current Windows Phone 8 handsets will receive the next major version of the operating system.
See the table at Microsoft here.