I personally used the "Length of Reign" rules not for the reign itself (because I felt it conflicted with monikers like The Young and The Old), but rather for determining how many years non-reigning heirs survived past adulthood. This way, if everyone is popping out 6-9 heirs who survive past adulthood, you can then try to determine which of them, if any, managed to survive long enough to produce children instead of merely an ever-expanding fractal.