How long have you and he been in a relationship?
Student or Learner
Does that mean "how long you and he are in relationship"?
I am confused...
Did you correct my question or is it the answer?
not a teacher
The question "How long were you seeing him (for)?", often means something like "How long were you dating him?".
However, it might be asking how long you had been seeing (i.e. consulting) a particular doctor, or something similar.
This is past tense. It doesn't mean "How long have you and he been in a relationship?". But if you change "have" to "had", that is one possible meaning.