To be fair Fergie is an outlier but it really should be a 19 year phase for Fergie if he's going for Del Bosque at 12.I don't really understand how they've decided who got what Imperial Phase.
Why does Del Bosque have a 13 year phase and Fergie only a 9 year one, when Fergie won more major trophies in his "diminishing returns" phase than Del Bosque did in his entire career? He had a six year gap in the middle of his Imperial Phase without winning any trophy while Fergie never went more than a year. Doesn't make sense to me at all.
What I did find interesting though is that, Fergie aside, most mangers really only have about 10 years at the peak of their powers once they start winning big trophies.