It always takes longer than you expect, even when you take into account Hofstadter's Law.
First coined in Hofstadter’s book Gödel, Escher, Bach, this law nods at the difficulty of accurately estimating the length of software projects. It’s self-referential in a way that Gödel especially would have appreciated. Indeed, advice to rising software managers commonly suggests adding a “buffer” to their estimates. This law suggests that the buffer will never be enough. Zawinski’s Law may provide an explanation for this phenomenon: even when developers complete their tasks within the estimated time, software bloat will cause the project to expand and then overflow the buffer.