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