If you check the Microsoft Support pages for Windows Phone 8 updates, at first glance it makes for rather grim reading, as it suggests that Microsoft's expectation for the OS is a straight 18 month lifecycle. More specifically, it reads:
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.
The end date capped for Windows Phone 8 is the 8/7/2014 (adjusting the US date), which means that Windows Phone 7.8 will actually have slightly longer to live, becoming unsupported on the 9/9/2014. That's what happens when you have 18 month life cycles for your mobile products.
The near certain reality for all of this is that it shouldn't matter much at all if you're in the Windows Phone ecosystem. Microsoft copped some flack when jumping from Windows Phone 7 to Windows Phone 8 because it underwent a radical code base shift, making Windows Phone 7 much less compelling as developers were enticed onto the new platform. Windows Phone 8 is much more closely tied into the core Windows family of products, and as such it's much more likely that Windows Phone 9 would share that codebase — and that's ignoring the fact that the next release of Windows Phone 8, codenamed "Blue", should get its own 18 month support cycle anyway.