“a dog's age”别理解成“一条狗的年龄”，其正确的含义是好久；很长时间。

【例句】
I haven't seen you in a dog's age. How's life treating you?
我很长时间没有看到你了。你最近过得怎么样？

He hasn't had a night out with his friends in a dog's age.
他多时未和朋友们在晚上出去玩了。