Question

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

Answer 1

There is no set duration. Queen Elizabeth II is now the longest serving British Monarch. She has reigned from 6 February 1952 - to the present = 63 years as of 2015. 

Lady Jane Grey, born 1536/1537, was known as the nine-day queen!

Answer 2

Until they die. A monarch rules until they die, get deposed, or step down