What is 「長い」 modifying in the sentence 「もう日本に来て長いんですか」?

Question

NOTE: The title of the post is regarding the most bugging part of the sentence to me (長い), but my problem is with the sentence as a whole.

I came across a sentence in my grammar exercises book I can't quite understand. I get the general meaning of the sentence, but I am unable to identify what grammar is used here. The original passage is as follows (the troubling sentence is in bold):

［状況］リーは1っか月前に日本に来た。

日本人: リーさんは日本語が本当にお上手ですね。もう日本に来て長いんですか。

I would translate it as:

Japanese person: Rii, you are really good at Japanese. Has it passed much time since you came to Japan? (not sure about the second sentence)

Assuming my translation is right, I would rather say something along the lines of:

日本にきてから長い時間が経ったんですか。

I can't figure out the function of もう and 長い alone, not modifying any noun.　Any thoughts on this?

よろしくお願いします。

Answer 1

「日本にきてから長い時間が経ったんですか。」

is almost as same as the original sentence :「もう日本に来て長いんですか。」.

This 「もう」 is "explicitly" intensifying 「長い(時間が経った)」 since the author is surprised at proficiency of Rii's Japanese and they want to make sure how much time has passed(estimating it should be/may be very long!). 　

Technically speaking, your sentence:「日本にきてから長い時間が経ったんですか。」may be implicitly intensifying 「長い時間が経った」, but it is difficult to be known without context.

The definition of 「もう」 from 大辞林

① 時間や程度が、ある基準や節目を超える意を表す。もはや。 「 －あれから一年たつ」 「 －これ以上食べられない」

• もうあれから一年たつ : "It's been already one year passed since then."

• もうこれ以上食べられない : "I can't eat any more."

Without 「もう」 can be the same as my translation. However, with「もう」it emphasizes it has already been beyond some "standard/capacity".