It seems common that development tasks or projects take much more time than expected. My manager recently pointed these out to me:

The 90/90 Rule - “The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.” — Tom Cargill, Bell Labs

Hofstadter’s Law - “Hofstadter’s Law: It always takes longer than you expect, even when you take into account Hofstadter’s Law.” — Douglas Hofstadter, Gödel, Escher, Bach: An Eternal Golden Braid