The tenure of a Prime Minister varies depending on the country. Here are some examples:

• United Kingdom: There is no fixed tenure for the Prime Minister in the UK. The Prime Minister serves at the pleasure of the monarch and can be removed from office by the monarch at any time. However, in practice, the Prime Minister serves until they resign or are defeated in a general election.

• India: The full term of the Prime Minister in India is five years, which coincides with the normal life of the Lok Sabha. However, if the Prime Minister loses the confidence of the Lok Sabha, they must resign or the President can dismiss them. The Prime Minister must also become a member of parliament within six months of beginning their tenure and must enjoy the confidence of the majority in the Lok Sabha.

Its worth noting that the tenure of a Prime Minister can be affected by factors such as a vote of no-confidence, resignation, or dismissal by the President or monarch.