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.
Well Australia doesn’t have a president. But in theory there’s nothing stopping someone from being prime minister for infinite time, provided their party doesn’t stab them in the back or their party doesn’t lose the election.
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.