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

Hi, Rashad, and welcome to the G.E,

@Rashad posted:

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.

Yes, both are grammatically correct. The first one works much better in American English. See here: