Is the question right or wrong? If wrong, please how can I ask about how long they haven't seen each other im the past.
Q: How long have you hadn't seen each other?
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No, your question isn't correct.
In terms of natural English the present perfect in negative sentences sounds okay mostly in progressive constructions. @Solarflare's options "How long has it been since you last saw each other?" and "When was the last time you saw each other?" are the ways most native speaker would say it.
Remark: I didn't place emphasis on what sounds natural in the first place, only on the constructed sentence from the point of view of grammar.