How long does it take to cure PE

I have experience PE for about 4 months now and have started practicing the start stop procedure with my gf while using vaginal intercourse. I find that it helps very little so I started the start and stop by myself by means of masterbation and I seem to last longer doing it this way but cant carry this into my sex life.

My question is how long does it take to where Ill be able to prolong my ejaculation during vaginal intercourse?

I dont masterbate excessivly and I have sex around twice aweek