Fermat's Last Theorem took 358 years to prove

In 1637, Pierre de Fermat wrote in the margin of a book that he had found a 'marvellous proof' that no three positive integers a, b, c can satisfy a^n + b^n = c^n for any integer n greater than 2 — but that the margin was too small to contain it. The problem resisted every mathematician for 358 years until Andrew Wiles of Princeton spent seven years in secret solving it, announcing a proof in 1993. That proof contained an error; he fixed it in 1994. The corrected proof runs to over 100 pages and draws on mathematical techniques that didn't exist in Fermat's lifetime, suggesting Fermat almost certainly never had the proof he claimed.

Source: Singh, Fermat's Last Theorem (1997)