Standards of Care requires 3 months if you live in the US. It does not state how often you have to meet. Any therapist who gives one to you before then should have their license/quals pulled because even three months is not that long and the standards of care are in place for a reason. In fact they are gambling with their licenese/quals if they don't follow the standards of care. They need to ensure that it is the right thing to do and that the person they are giving it to is not on some kind of fad or giving them a story they made up and are trying to believe. Yeah, people still could do something like that..... but odds are in that time a trained therapist can weed out the crap and get to the bottom line unless they feel a need to go longer based off something that is said, etc.
How often you meet is up to the therapist and how they feel about your situation and stage in transition. I meet my therapist every 3 weeks and it's just to see how things are going. Before my T letter it was every 2-3 weeks. Each appointment is only an hour. The factors she took into consideration and has voiced to me verbally are age, how long I had been living as a man pre-T (and any circumstances that may prevent full-time), knowledge of transition and my past. There is not exact formula because everybody is different. Just be honest and let their medical/counseling experience do the rest. If they feel and agree with you it's the right thing at the right time, they will suggest your appointments be more spread out. If you are the first GLBT person they have seen....then you may be at the mercy of the therapist for everything regardless.