It was probably somewhat longer. If you want to go back a long way.... Every time I have occasion to drop in on the Deputy Minister of Finance, which I don't do very often any more, I marvel at the row of pictures on the wall of people who served as Deputy Minister of Finance between, if I recall, 1870 and 1891—you know, 20 or 25 years in a job. Nobody does that anymore.
I think the tenure probably was longer in the fifties and sixties, although my recollection of that is somewhat limited.
Opinions will differ. Wayne Wouters is quite right that you really hit your stride after the second year. It doesn't mean that you're useless in the first two. If you're an experienced official and you've been a deputy elsewhere, there are a lot of things you can handle, because they require a general knowledge of how government works rather than the detailed knowledge of that particular portfolio. Nevertheless, you're better in your second year than you were in your first, and you're probably better in the third than you were in your second.
So I'm with you entirely on the desirability, but you have to recognize that a prime minister—and these are prime ministerial appointments—can face all sorts of situations that make it necessary to move people, even while recognizing that it's not a good idea.