We normally aim for about 5 years, but it depends on when support gets dropped on other OSes since so much of the code is shared.
I do not think You could go on and on without separating the two areas because Linux takes out new kernels and new Xserver much more rapidly than are the changes in Microsoft house.
In any case I think that would be sufficient a few revisions in way to update the code for new versions of Xorg and the kernel. Ultimately, if NVIDIA does, why you should not be able to do so?
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