How long have you known your best friend?
I have known my best friend for _______.
How long have you studied English?
I have studied English since _______.
How long have you been interested in (any hobby or interest)?
I have been interested in _______ for _____.
How long have you lived in _____?
I have lived in _______since _____.
How long has your _____ worked at his/her job?
My _____ has worked ____ since________.