The tradition began in February 2003 when Donald Rumsfeld, the US defence secretary, in a now much mocked prediction, told soldiers based in Europe that the Iraq war "could last six days, six weeks. I doubt six months."
Three years and eight months later, 2,700 American troops have been killed and 142,000 are still deployed in an increasingly rebellious and violent Iraq, where officials are nevertheless still talking in terms of a six-month turnaround.
"This is a decisive period for everyone and everyone knows it. The next six months will determine the future of Iraq," Casey said on Thursday after a meeting with top multinational and Iraqi military commanders.
His warning was seriously meant, but might have carried more weight if it had not so obviously echoed a litany of similar predictions over the years.
In January 2004, nine months after the US-led invasion and at a time when the rising against occupying forces was gaining in strength, Tony Blair, the British prime minister, visited the Iraqi city of Basra.
"The important thing is to realise we are about to enter into a very critical six months," Blair said on his flight home.
"We have got to get on top of the security situation properly, and we have got to manage the transition. Both of those things are going to be difficul."t
Things did indeed prove difficult.
By 2005, the struggle to win the fight in Iraq had begun to undermine the credibility of US President George Bush's war plan, prompting some of his own domestic supporters to seek quicker results.
"The next six months will be very critical," said Nebraska's Senator Chuck Hagel, a leading legislator from Bush's Republican Party, in August 2005.
"Not just the constitution writing, referendum, the election, but also within that six months' period we are going to see whether the Iraqis are really going to be capable of defending themselves," he said.
As might be expected, Bush's opponents were also impatient.
In December 2005, Senator Joseph Biden, a leading Democratic opponent of the war, told CBS television "the next six months are going to tell the story".
After many delays and much violence, an Iraqi government was duly elected and homegrown security forces began to take the fight to anti-US forces and militia groups opposed to the US-backed government.
Nevertheless, sectarian revenge between Shia and Sunnis erupted, and the death toll from the violence continued to rise.
In June this year, the US ambassador to Iraq, Zalmay Khalilzad, said in an interview with Germany's Der Spiegel magazine that "horrible mistakes" had been made during Iraqi reconstruction efforts and must be fixed.
"The next six months will be critical," he declared.
Highlighting two events
Mounzer Sleiman, a Washington-based Lebanese political analyst, said the term six months is usually used to mark the period between two important dates.
"When Khalilzad says the next six months will be critical, he is referring to the period between June and November's mid-term elections in the US, which would greatly affect George Bush’s position," he said.
Sleiman said the situation between the US and Iran directly affects the situation in Iraq.
"I think Casey’s use of the term six months has something to do with Iran's nuclear case. As we all know Iran enjoys great influence on key Iraqi players, so he is somehow saying if the US and Iran remained at odds regarding the nuclear issue, things in Iraq might get more complicated."