I haven't seen you in years.
I haven't seen you in a long time.
I haven't seen you for years.
I haven't seen you for a long time.
The first two sound natural to me. The last two, not so much. They're not grammatically wrong, but I wouldn't use them.
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The first two sound natural to me.
The last two, not so much. They're not grammatically wrong, but I wouldn't use them.
CJ