Kurzversion: Long time no see.

Langversion: I haven't heard from you in a long time.

z. B.

Wow... Long time no see but absence makes the heart grow fonder I guess... How ARE you???

Long time no hear

#3 would be at best some sort of inside joke, as far as I'm concerned.

#1's I haven't heard from you in a long time. is also what I would say when I get an IM, e-mail or written correspondence (NOT: haven't read from you.)

Long time no hear
FWIW, I've said that more than once; everyone knows '...no see.', so '...hear' is understandable, I'd say :-)

Long time no hear is in some sense understandable, I suppose, if that's the standard by which it's to be judged. As I suggested, it may be some sort of in joke or jargon for some group. Otherwise, for me, it sounds like it's translated or (quite) substandard. Long time no see., as suggested in #1, might be a better choice (although I don't like it much here.)

thundt, like stonehenge, does not seem to be a reliable authority for English idiom, in my opinion (to put it mildly).

AE

I'd also say that Long time no hear is pretty common, and I've certainly used it myself in colloquial written form. I'd say it’s no more or less substandard (#6) than Long time no see!

I've worked with people I've never met; if one calls after a break of several months, 'long time no hear' is not inappropriate I'd say. :-)

There could well be different perspectives here for BE/AE, age group, immersion in social media, etc., etc.

I consider Long time[,] no hear to be a/an (inside) joke, substandard, foreign, or some such thing.

age group, immersion in social media, etc., etc.

FWIW: age group - hmm, elderly; immersion in social media - none; but as I'm British, I do enjoy puns, groaner jokes and shaggy dog stories etc. :-)

OT: Some English cousins are coming to visit in a few days. Normally, the weather here isn't that bad at this time of year, but they are in for a cold surprise this weekend. I'll see what they have to say about some of our BE/AE questions, if they're in the mood. Unlike most of us in this forum, their English has not been influenced by any exposure to German (except as tourists).