"He is long gone." is a bit colloquial in English but I believe it means the same thing as "Han er længe væk." But it's not accepted as correct.
To me ‘he is gone for a long time’ (or he has been away/absent for a long time) implies you are expecting him back. Whereas I would say ‘he is long gone’ just means he left a long time before you asked, there is no expectation that he will return.
Is laenge only used for length of time? I did "He is far away" but it was not correct. I guess that would make sense, though, saying he is 'long gone'.
As far as I've heard "han er langt væk" refers to distance and "længe" refers to the time
I put He is away for a long time. This should be accepted as away is even shown as one of the meanings of væk in the drop down list and it means the same thing.