

Agreed that PM is different, they don’t weild the same power the US president does. But in terms of time, Robert Menzies was pm for a total of 18 years and John Howard was pm for 12, so you can be for a long time. Politics have just been much less stable in the last 20 years.
Not that I’m aware of? Technically the king/queen is head of state and that term is until death, can’t get much longer than that.