It implies that you have been away from some place for a time and have now returned to that place.
Student or Learner
What does it mean "How long have you been back"?
not a teacher
How long have you been back?
How much time has passed since you returned?
a) I went to France and thought I'd like to live there, but I changed my mind and came home.
b) How long have you been back?
a) About three months.