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