1) Kopę lat!
Or:
2) Nie widzieliśmy się kopę lat. (You say that second one when you want to build whole sentence, with predictate, but like in English, you very rarely use that, I mean in phrase "long time no see" there isn't any predictate either, but it's common in English too)
And it would be better to ask : "Jak powiedzieć 'long time no see' po polsku?"
Don't get me wrong, "Jak powiesz 'long time no see' po polsku?" is correct, but I think my version is more general than yours and more appropriate in this situatin, when you're asking all community, not just one person (because you've asked: "How do you say (...)?", and it's good, but "you" was singular.)