This expression is an abbreviated statement. It would usually be written in long form as "It has been a long time since I have seen you." The expression is not grammatically correct, and it is a lazy way of saying something, but it is often used with a smile on your face, and it is acceptable in informal English, but not acceptable in formal English.

Yes, it is perfectly correct. It is a literal transliteration from a Chinese saying.

This is 'pidgin English', a very basicform of English used when trading with non-English speaking people...(probably in the 18th century). Interestingly, the word 'Pidgin' is an English person's version of a Chinese person's version of the word 'business'.

This expression is an abbreviated statement. It would usually be written in long form as "It has been a long time since I have seen you." The expression is not grammatically correct, and it is a lazy way of saying something, but it is often used with a smile on your face, and it is acceptable in informal English, but not acceptable in formal English.

Yes, this saying is right. :)