Parkinson’s Law states that “work expands so as to fill the time available for its completion.”
“Managers know that software development follows Parkinson’s Law: Work will expand to fill the time allote to it. (…) corollary to Parkinson called the Ninety-Ninety Rule, atributed to Tom Cargil of Bell Labs: ” The first 90% of the code acount for the first 90% of development time. The remaining 10% of the code accounts for the other 90% of the development time.”
Hofstadter’s Law states that: It always takes longer than you expect, even when you take into account Hofstadter’s Law.
A (somewhat joking) rule of thumb introduced by Hofstadter for figuring out an accurate time is to double the number and step up to the next higher units. For example, a job estimated at 1 hour can be accomplished in 2 days, while a 3-month project will take you 6 years.