Again, I think it varies. Speaking from personal experience, after 10 years of being a deputy, it depends very much on which department you're in. Some departments are very complex, and it does take a period of time to get a good understanding and be able to lead that department. For others, much less time.
I think another key issue in this area, and one of the most important aspects of the job, is the relationship between the deputy and the minister. You could be the best deputy head in the world, but if for whatever reason the relationship isn't good with your minister, is it productive to continue to try to operate in that environment? With a fixed tenure, you are potentially taking away that option of saying this is not working as a team, what can we do about it? How do you deal with that under a system of fixed tenure?
Again, I think you should talk to the PCO officials about all the issues around it. They are much better positioned to talk about that than I am.