Helo, Mr. Wen.
(1) This is only my opinion. It is not a generalization.
(2) I do not think that this expression is used too often nowadays
in the United States -- especially by the youth.
(3) I personally think that it would be insulting to say this to an
Asian person -- since long time no see is a direct word-for-word
translation of the Mandarin Chinese expression. It would imply that
the Asian person does not understand standard English.
(4) I am shocked (shocked!!!) that, according to you, American
teachers in China say this to their students. In my opinion, it shows
a lack of sensitivity on the part of those teachers -- who should conduct
themselves as role models and ambassadors of good will for my country.