Parkinson's law states that "work expands so as to fill the time available for its completion."
It was first articulated by C. Northcote Parkinson in the 1957 book of the same name as a result of extensive experience in the British Civil Service. The scientific observations which contributed to the law's development included noting that as Britain's overseas empire declined in importance, the number of employees at the Colonial Office increased.
It can be seen by most people that the more time you have to do something the more the mind will wonder and new problems will develop (usually by people who want you to do some work!).
"Parkinson's law" is also used to refer to a derivative of the original relating to computers: "Data expands to fill the space available for storage"; buying more memory encourages the use of more memory-intensive techniques. It has been observed over the last 10 years that the memory usage of evolving systems tends to double roughly once every 18 months. Fortunately, memory density available for constant dollars also tends to double about once every 12 months (see Moore's Law); unfortunately, the laws of physics guarantee that the latter cannot continue indefinitely.
Part of this page is based on the entry Parkinson's law taken from the Jargon File.