Hmmmm, another difference in UK [or British] and North American usage, I guess? Because "He's been a long time." just sounds wrong to me. As Neil said, it's missing something. He's been WHAT a long time? He's been in France a long time. He's been a doctor a long time. He's been in jail a long time. He's been a long time advocate of animal rights. The Brits here all seem to agree that "He's been a long time." is perfectly acceptable. Perhaps one of them could tell us just what it means?! He's been WHAT a long time? Because I would not know how to interpret that sentence! Always fascinating to discover differences in usage that we didn't know about! Cheers.

Yes, it's absolutely correct. Of course, the present perfect tense is used here, because the timeframe is 'up to now'. Later, we can talk about the same situation in the past; for example, 'You were a long time at the shops.'

For me, 'He's been a long time' is an absolutely correct and natural remark to make in everyday conversation. I see that another UK member also says this, but a US member says it isn't normal. I wonder if this is a regional difference? Comments from other native speakers would be welcome!

I agree with what's been said so far. As a native speaker of North American English, "He had been a long time" sounds strange to me. I would say either: "He has been a while" (in a context where it is understood he is gone and I am waiting) "He has been gone a long time"

No, you would normally say "It has been a long time..." just like what you wrote after the word "mean." "It has been a long time since he..." is common in everyday English. The phrase "He has been a long time waiting" is okay, but it is rare. Instead, "He has been a long time (long-time)..." is often used to describe a person's job, profession, advocation: He has been a long-time writer... She has been a long-time advocate of a clean environment... Mark has been a long-time student of English...