Westheimer's Rule (related to Murphy's Law) states that in order to figure out how long a task will take, take your best estimation, double the figure, and use the next larger order of measurement. Therefore, a one-hour job will take two days.
I first became aware of this rule of the universe years ago, and I have seen it come true in the real world.
In the early 1980s a clinic I was working at, funded by Federal Rural Health Care money, wanted to install a computer system, mainly on the basis that "everyone else has one." It was supposed to make our quarterly reporting to Uncle Sam easier. I pointed out that the software for this data mining did not then exist and would have to be created from scratch. The clinic went ahead anyway. The computer guy gave us a one-month estimate for complete set-up. It took two years.
When I was in the Department of Geologic Sciences at the University of Washington we arranged to collect a 9 foot by 8 foot slab of rock in the Cascade Mountains that was about 50 million years old and had bird and animal tracks in it. My professor estimated that the job would take two days. It took four weeks almost to the hour. We needed more rock wedges, they sent out the wrong ones twice, it took a day and a half to get the slabs in a truck, etc.
Currently I am working in the medical transcription department of a large hospital. We have been trying to get a new computer system installed for years, the first meeting on the subject with a consultant having been scheduled for 09/11/01 (and had to be postponed). They said it would take three months. Right now, two computer companies later, it looks like September 2007 will be about right for start-up (six years).
Donald Rumsfeld said Iraq would take less than six months. Therefore, the last US military will leave there after twelve years (2015).