Alot depends on the person and the length of time that patient was deficient.

In Japan in the '70's there was a severe epidemic of low B12 from a drug commonly used there (Vioform). Many of those patients did not get better, so Japan leads the world in testing and treating low B12 even today.

This link explains more:
https://en.wikipedia.org/wiki/Subacu...of_spinal_cord

B12 is used by the body for many things....so there is a priority of sorts when starting supplements. But there has not been much in the way of research describing recovery %'s or timing.
And it may be that one would need B12 supplements for life. If the original cause is not found for the deficiency, I believe one should assume,a life long need to supplement this.