Hello, Frenchy's Rule. Although I'd understand you if you said this, I don't recommend that you mix these two. People will probably think that you've made a mistake and try to correct you. That can get annoying.
Both expressions can be used (separately; not combined!) in similar circumstances, but note that the set expression Long time no see (which at its origin was an imitation of a non-native's attempt to express this idea--see here) is much less standard than It's been a long time (since we've met).
Long time no see is mostly conversational; you would not generally use it in formal or business writing.
"Long time no see" is very informal and, in fact, can be offensive to Asian-Americans because the expression originates from the broken English of Asian immigrants, especially Chinese. When I was a child growing up, I've heard Chinese immigrants in New York say long time no see firsthand, especially when I've visited a Chinese restaurant in my old neighborhood years afterwards; the restaurant owner knew my family.
You're better off saying, "I haven't seen you in such a long time" or, even better, "I haven't seen you in ages."