Estimated and Actual Duration of Projects
When the first world war broke, German military leaders estimated it would last approximately six months. But over three years and three months later Kaiser Wilhelm fled the country and Germany capitulated.
When the largest land war operation in history, operation Barbarossa, started on June 22, 1941, German leaders promised the troops would be home by christmas. Three years and ten months later Hitler committed suicide in his underground bunker.
(Actually, some historians claim that the first world war wasn't concluded until the second world war ended - there was merely a 21-year armistice.)
The Vietnam war ended exactly three decades after Hitler's suicide. But six years and three months earlier Nixon had promised congressman Donald Riegle (R-MI) that the Vietnam war would end six months after Nixon assumed office. After all, Nixon had a "secret plan", just like George W. Bush has today.
And speaking of Bush Junior, it's now three years and eight months since he declared the "mission was accomplished" and "major combat ended". Since then, the guerillas have fulfilled Bush's request to "bring'em on", resulting in over twenty-folding the U.S. body count.
So, over-optimistic programmers are just like politicians and military leaders: given their estimate of how long the project will take, if they haven't succeeded in twice that time they will fail and at least six longer until they realize it themselves.
(To be fair, there have been also realistic estimates in both venues. Churchill, for example, never promised a quick and pleasant end to the second world war.)