According to a Microsoft support page, support for both Windows Phone 7.8 and Windows Phone 8 ends in 2014. Basically, this means that anyone running either of those two operating systems will not receive any security updates after 2014.
The end of support dates are:
Windows Phone 7.8: 09/09/2014
Windows Phone 8: 08/07/2014
Microsoft also attaches the following review note next to both operating system:
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.
It is unclear whether a future Windows Phone 8.5 release will extend this support date but considering Windows Phone 8 came out in late 2012, those on 2 year contracts will be able to upgrade at around the time that Microsoft pulls the plug on support.