Indefinitely, hopefully. There will always be video cards that either aren't powerful enough or just aren't supported. And there'll probably always be users (like me) who have both powerful and supported graphics but that don't want Unity or Gnome Shell.
Moving to a scenario where you're forced to use a UI that requires accelerated graphics would be a dark day for Ubuntu.
It's important to note that this doesn't just apply to old graphics hardware but also new hardware that doesn't have a Linux driver (or at least one accessible in a given release of Ubuntu).
I suspect the actual kill-date will be determined when Gnome announces it'll stop supporting the 2.x interface.