It has been a long time since I have seen her.
is an incorrect sentence for me.
The same tense cannot be used twice in the same sentence in order to be the time reference for the other (but the same) tense.
Could be said
It has been a long time since I saw her.
If one want to use the present perfect in the latter part (That the latter part carries either meeting or having-a-word connotations) then the indefinite tense must be used in the first part of the sentence according to logic of the Sequence of Tenses.
Let me try to develop the most liking form for me
Let me look at these forms, that could be correct, first
It is a long time since I haven't been seeing her.
It is a long time that I haven't seen her, since the last time I saw her. ("the last time I saw her" is a bit redundant here)
It is a long time that I haven't seen her, since.
It is a long time that I have seen her.
And at length one could maybe say
It is a long time since I have seen her.
I wanna say that I prefer the second last sentence. It means this one below in blue
It is a long time that I've seen her.
I wonder how many people that either learn or speak the language prefer the same sentence.