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 had your pet? how long have your father known your mother? how long have your family lived in your flat (house)?

1 How long have you studied at your school? 
I have  studied for 8 years  at my school.
2 How long have you known your best friend?
I have   known my best friend for 10 years.
3 How long have you had your pet?
I have had my pet since I was 6.
4 How long has your father known your mother?
He has  known my mother since his childhood.
5 How long have your family lived in your flat (house)?
We have lived in our flat for 12 years.