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

Generally, all monarchs rule for life or until they abdicate.

Because they become a monarch after their parent's death (the previous monarch), they could be crowned only in their 50's and consequently rule for about 20 years.
However, it is not unheard of that very young people become monarch, such as Austrian Francis Joseph, or Queen Elisabeth, and then they can rule for over 60 years.