In the United States, either 2 elected terms or a maximum of 10 years. For example, if Obama leaves office for whatever reason at the end of 2010, Biden will serve during 2011 and 2012. He would then be eligible to run for president twice more, bringing his total to 10 years. If he were to take office anytime before 2010, he would only be able to as president once.
Other countries have different rules regarding term limits.