I have not found anywhere where BSA states how many terms in succession a Scout may retain the same leadership position -- whether that's SPL, Patrol Leader, Scribe, Historian, etc. Is there a guideline for that?
In my opinion, Scouts that have already held the position of Patrol Leader (for example) for one year need to move on. It is my understanding Boy Scouts is designed to develop leadership skills in boys. Boys that have held such a position already know how to lead a Patrol (or whatever duty they're responsible for) and should now be in the position to help teach the next Scout ready to step in to the role.
Obviously, my Troop thus far has no limits on the number of times you can be re-elected (or re-chosen, as the case may be) and it irks me a little (a lot).