It basically means, I think, that the time you have available for a job is how long it will take to do that job. If you twice the time, you’ll take twice the time to do the work.
You should be working as quickly as competently possible to stay sharp.
"Work expands to fill the time required for its comtemplation."
Parkinson's law
Wikipedia
Parkinson's law can refer to either of two observations, published in 1955 by the naval historian C. Northcote Parkinson as an essay in The Economist:[1]
"work expands so as to fill the time available for its completion",
the number of workers within public administration, bureaucracy or officialdom tends to grow, regardless of the amount of work to be done. This was attributed mainly to two factors: that officials want subordinates, not rivals, and that officials make work for each other."
The first paragraph of the essay mentioned the first meaning above as a "commonplace observation", and the rest of the essay was devoted to the latter observation, terming it "Parkinson's Law".
First meaning
The first-referenced meaning of the law – "Work expands to fill the available time" – has sprouted several corollaries, the best known being the Stock-Sanford corollary to Parkinson's law:
If you wait until the last minute, it only takes a minute to do.[2]
the Asimov corollary to Parkinson's law:
In ten hours a day you have time to fall twice as far behind your commitments as in five hours a day.[3]
as well as corollaries relating to computers, such as:
Data expands to fill the space available for storage.[4]
"Parkinson's law of triviality" is not to be confused with Parkinson's law.
The law of triviality is C. Northcote Parkinson's 1957 argument that people within an organization commonly give disproportionate weight to trivial issues.[1] Parkinson provides the example of a fictional committee whose job was to approve the plans for a nuclear power plant spending the majority of its time on discussions about relatively minor but easy-to-grasp issues, such as what materials to use for the staff bicycle shed, while neglecting the proposed design of the plant itself, which is far more important and a far more difficult and complex task.
The law has been applied to software development and other activities.[2] The terms bicycle-shed effect, bike-shed effect, and bike-shedding were coined based on Parkinson's example; it was popularized in the Berkeley Software Distribution community by the Danish software developer Poul-Henning Kamp in 1999[3] and, due to that, has since become popular within the field of software development generally.