Hofstadter's law

A very interesting adage. I'm not sure how you pronounce the guy's name but the saying goes like this:

It always takes longer than you expect, even when you take into account Hofstadter's Law.

Do you see the recursion in it? This is a self-referencing adage by Douglas Hofstadter. It is often cited among programmers, especially in discussions of techniques to improve productivity. Hofstadter's Law is a profound statement of the difficulty of accurately estimating the amount of time it will take to complete tasks of any substantial complexity.

Even after you have taken Hofstadter's Law into account, by Hofstadter's Law you must still take Hofstadter's Law into account, and this remains true no matter how many times you have already applied Hofstadter's Law.

It is therefore impossible to ever fully take Hofstadter's Law into account, at least in finite time. While some recursive functions do seem to eventually converge on an approximate solution (e.g., x/2 approaches zero when its output is repeatedly fed back into it), anyone who has attempted to manage a software project can testify that this does not seem to be true of Hofstadter's Law; if anything, estimates that take Hofstadter's Law into account even partially seem to have an infinite, or at least very large, upper bound. Thus, Hofstadter's Law implies that it is not merely difficult to estimate a project—it is potentially infinitely difficult

A (somewhat joking) rule of thumb introduced by Hofstadter for calculating an approximate 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.

PS: finite difficulty: requires that the extra difficulty decrease quickly enough for the recursive series to converge to a finite value in finite time).