150 years
Saudi Arabia is still an absolute monarchy, and has been since 1992, when The Basic Law of Government adopted in 1992 declared that Saudi Arabia is a monarchy ruled by the sons and grandsons of the first king, Abd Al Aziz Al Saud.
Oman has been a country for about 800 years.
Sweden has always been a monarchy.
Present-day Lebanon never was a monarchy; it has always been a republic.
Iran is not a monarchy; it has no king. It is a theocracy, ruled by the Ayatollah Khamenei as Grand Ayatollah. However, prior to 1979, Iran was a monarchy and had been for over 2000 years.
Sultan Qaboos has been in charge since he overthrew his father in 1970
An ancien régime is a system of government which has been long supplanted by another, particularly a state of feudalism with an absolute monarchy supported by the doctrine of divine right with the explicit consent of an established church.
Australia has been a constitutional monarchy since Federation, which occurred on 1 January 1901.
Oman had a long running civil war (although it was never termed such) in the 1950's to mid 70's in the interior southern Emirates.
If he is powerful enough he can continue for a long time, but inevitably his subjects will revolt and usurp him. I saw this happen in Oman in 1970 when Sultan Said bin Taimur was evicted by his son Qaboos and other senior army officers. This was an almost bloodless coup and Said was allowed to retire to UK with his wives and personal property.
Sweden has never been ruled by a dictator, we used to be an absolute monarchy however meaning the king had pretty much full power of the country, but Sweden today is a constitutional monarchy and has been so for a pretty long time. The last king of Sweden who had political power was Gustaf V (ruled from 1907-1950) but it was still the parliament who had the most power.
Poland has not been a monarchy for a very long time.