How long have you studied at your school? How long have you lived in this citi? How long have you known your best friend?

 How long have you studied at your school? —  I have studied at school for five years.

 How long have you lived in this city? — I have lived in this city since my childhood.  

How long have you known your best friend? — I have known my best friend for five years.

How long have you had your pet? — I have had my pet for two years.

How long have your father known your mother? — My father has known my mother from school.

How long have your family lived in your flat (house)? — My family has lived in our house for twenty years.