As a developer, I think that not for long after 8 gets released. Supporting both 6.0 and 7.0 is a pain in the ass because there are too many differences that need to be handled.
But in the end, it will ultimately depend on the number of users who will continue to use 6.0.
Many apps now support ios 7 and some of them require only ios 7 but what about ios 6? How long can ios 6 live? App support is rapidly getting smaller and smaller and my iphone 4 is still on 6.1.3. How much time does ios 6 has?