a dog's age

a dog's age 翻译为：好久, 长时间

这是俚语吧
为什么会翻译成这个意思呢？俚语貌似都有一定的“典故”呢

我就是想知道基佬把dog 用在这里，为什么翻译成了“长”呢？

请高人指点

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