English Translation
A wonder lasts but nine days.
Kind of a strange translation
They are proverbs in Japanese and English. They mean the same but with different days required. Take a look at 英辞郎.
According to this article. 75 days in the Japanese proverb refer to one season. We used to divide a year into five seasons (four seasons in the modern era + one rainy season during June/July). So, one season is approximately 75 days,
A gossip lasts less than one season. That’s the meaning of 人の噂も七十五日.