If you've ever done any project planning, you may know that Douglas Hofstadter, in his book Gödel, Escher, Bach, formulated an important law, named Hofstadter's law:
It always takes longer than you expect, even when you take into account Hofstadter's Law
In practice, this recursive law is very hard to apply. I use a non-recursive variant myself that also takes into account project planning:
A carefully planned development project takes π times longer than you think. If you don't plan carefully, it will be π2