The first is fine; the other two don't work.
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Anonymous Hi,The situation I’m asking two of my friends the following questions. Which sentences sound natural?1. How long has it been since you last saw each other?2. How long haven't you two met? (intransitive verb)3. How long haven't you two met each other? (transitive verb)Thank you for your reply.2. and 3. don't make much sense. You may ask instead: 'Whe