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1 how long is it since you last saw him?
2 tomorrow it will have been ten years since I saw him
3 It has been a long time since I saw him
4 It is a long time since I have met him
They look fine to me. I might suggest for 4. It has been (or It's been) a long time since I met him.
I generally sense that the "have" in the original "have met him" should be reserved for things that had a duration in time, when they happened. "It's been a long time since I have eaten with him" - because eating takes time, whereas when you met him was an exact point in time.
I don't know why we must use present perfect in "It's been a long time since I have met him". Since the information is how long you've been without meeting someone. And to me the time you take meeting someone is insignificant compared to the time you've been without meeting. Why not to use the past simple?
are these sentences correct? tenses.... 
Linear Mode
