Translation:Nine months since I have seen you last.
is the word order here a result of 'seit'? or are we to suppose that some other preposition has been stated beforehand?
I think there's an implicit sentence with "Neun Monate." I'm not exactly sure what the collocation is, but something along lines of "Es ist neun Monate, ..." In other words, it's not "seit," but rather the construction of a sentence that makes the rest of the sentence like that.
Is this a natural English sentence? Does it mean the same as 'Nine months since the last time I saw you'?
I think it is slightly awkward in English because of the adverbial + tense, I'd much prefer your version.