As of August 2011:
So far, President Obama has taken 61 vacation days after 31 months in office. At this point in their presidencies, George W. Bush had spent 180 days at his ranch where his staff often joined him for meetings. And Ronald Reagan had taken 112 vacation days at his ranch.
Among recent presidents, Bill Clinton took the least time off -- 28 days.
To be fair, a presidential vacation away from the White House is not the same as a vacation for the average person. The president is still in contact with his advisers and on call for any emergency.