How many repetitions does it take to recover a function?

This is totally unknown. What if you have only one neuron surviving that does that function? Or 5? Or 10? Or 50? Is that enough to recruit more via neuroplasticity? What a fuckingly simple question, yet no one knows one damn thing about neuroplasticity. 
How many hours of action observation do I need to do to lay the foundation for moving functionality from a dead area to a new area? Then how many repetitions of actual movement do I need? If 10 million I would get a counter and start doing it. But we are completely in the dark. This is why I don't even do any specific therapy any more, I have a life to live and with no guidance it seems hopeless.
What I hate about this question is that I bet NO one in the world has even attempted to create a research project on this. Very simple; an fMRI to determine objective size of existing functional control area, then various numbers of repetitions, fMRI again to see increased size. You have an objective starting and ending place with objective intervention numbers. From that you create a fucking simple stroke protocol. And no one in the years since fMRI has existed has had enough brains to figure this out?