How long is an individual monarch's term of rule?

Until death. However, resignation or an overthrow can end their lifelong position early. Being monarchies are hereditary they are held by the family leader until their death. Following a death of a monarch, the line of secession is triggered and the next family member in line takes the throne. There have been cases of resignation called abdication such as Nicholas II in Russia. Overthrows of the government can also end a term early like with Louis XVI of France.