Comment by KineticLensman

Hofstadter's law [0]: It always takes longer than you expect, even when you take into account Hofstadter's Law.

Interesting, Hofstadter's original motivation for the law was the observation (published in 1979) about the then delays in computers beating grandmasters at chess (first in 1997 and now routinely_. This gives us a data point about 'just how long' things can take (in addition to the numerous examples from software projects). Nuclear fusion is still out there...

[0] https://en.wikipedia.org/wiki/Hofstadter%27s_law