To be honest, it could even work after the first time. How often do you have to do it depends on how much you believe in it and how much it actually works. If you can’t convince yourself to it and believe it works, it won’t. That’s how self hypnosis works. It doesn’t work thanks to the method itself, but because YOU believe in it.
For example, one man truly believed in self-hypnosis, and meditated-prayed mantras which went something like "I believe in my strengths(…) and that I will heal myself" - he suffered from skin cancer, something that couldn’t be healed in 1980’s, but he healed himself. How? Heck knows.
First start off seing if you really believe in it, then, according to how much you have to convince yourself to believe in it TRULY, adjust the rates accordingly. 3-4 times 10 minutes(between meals) is ok for me.

Cheers.
References :

How do you know if you are even going into the trance state during a SELF hypnosis session ??. A professional hypnotist can observe several physical signs and your responses to a list of other signs, do testing etc. and have a good reference to go by. When i work with a new client the first induction takes up to 45 minutes the second may take as little as 15 seconds the client usually tells you something like they just went down a long slide. and they feel very good. After that it gets better.
References :
professional hypnotist with 50 years experience-retired now.