Mainstream support for Windows 8 is currently scheduled to end on January 9, 2018. Microsoft has yet to reveal a support end date for Windows RT, although its official support for the Surface RT tablet is supposed to stop on April 11, 2017.
But what about its support for its two current Windows Phone versions, 7.8 and 8? Microsoft's support page shows that mainstream software support for Windows Phone 7.8 is supposed to end on September 9th, 2014. The deadline for support of Windows Phone 8 is a few months earlier, on June 8th, 2014.
At the moment, it doesn't look like Microsoft will extend mainstream support for Windows Phone 7.8 beyond its current deadline, mainly because it was made as an update for most of the phones that were originally for Windows Phone 7 and 7.5. However, there are some rumors that indicate there could be at least one more major software update for the OS. If that happens, the mainstream support date could change.
The deadline for Windows Phone 8 will almost certainly be extended when and if Microsoft launches a major update for the OS. That will likely come by the end of 2013 or early 2014 with the launch of Windows Phone Blue.
Source: Microsoft Support