Answer :

lrathburn
An individual monarch's term of rule can be in effect until their Death, they resign, they're overthrown, or If they commit a crime.
RedRicin
Monarchy is when your oldest son takes over as king. That would be either when you die, step down, or are overthrown. (in which case your son wouldn't get to be king but someone else would) There's not really a set amount of time a monarch rules because of this.