When you say "an ideal span," I assume that you are talking about a span of time. I assume that you are saying that there is some amount of time that is the right amount for one manager to stay in a given position. I would agree that this is absolutely true.
There are two reasons for this. First, managers who stay in one place too long might get set in their ways. They get a workplace set up the way they want it and they are unwilling to think about the fact that their way (which worked before) may need to change.
Second, managers who work with the same people for long periods of time may find that they are no longer able to get through effectively to those workers. The workers have heard everything the manager has to say and might start tuning the manager out at times. It is useful to get new blood in so the workers can hear a new voice, one that they are not used to and tired of.