1. How long have you known your best friend?
I have/I've (obie formy tu pasują) known them for x years/months. / I have known them since I was x.
2. How long have you had [something]?
I've had it for x years/months.
3. How long have you studied English?
I have studied English for x years/months.
Mam nadzieję że pomogłam :)
