I am in the not completely unusual position of having an eight year period between my Master's and PhD degrees. During this time I was gainfully employed in a completely unrelated and non-academic field.
I have now graduated from a PhD and am looking for work. Many good post-doc and academic jobs require three referees to speak on one's behalf, and to be competitive these references must surely be academics.
I had a great relationship with my Master's supervisor, including co-authoring papers together, and I believe that despite the 12 years since I was her student she could still speak credibly, confidently, and highly favorably on my behalf.
In the non-academic world it would stretch credibility to call upon a reference from such a distant point in the past. But for an academic position, I think things could be different.
What are the potential advantages and disadvantages of using her as personal reference? Are potential post-doctoral supervisors or hiring committees likely to discount her reference as stale and irrelevant?