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"In" or "for" with "a long time"?

witch one is correct ?

we haven't seen him in a long time.

we haven't seen him for a long time.



So I had this discussion with my professor about which one of them is correct, and he said "for" is the correct usage and "in" is wrong. but I'm 90% sure that they are both correct in this context. So I need a clear answer with detailed explanation about this question and please provide me with references so I can prove him wrong. Thanks.

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