Some countries may have a limit on the number of years, but many countries (including the United Kingdom) do not.
Prime Ministers are not directly elected. People elect a parliament. The parliament works out which party will lead government, and the leader of that party will be the prime minister.
A party can continue to govern as long as they keep winning elections. The leader of the governing party (and thus the prime minister) can change at any time.
See
http://wiki.answers.com/Q/How does a parliamentary system of government work