It's possible to transition with minimal or no therapy, something I don't recommend. If you think your transgender, most likely you are but that doesn't ensure there isn't an underlying problem. There are member on this site who have other normal conditions like schizophrenia, bipolar, multiple personality, depression not related to GD, relational issue, abused as a child and other issues. One should undertake enough therapy to ensure that if these conditions exist, they are addressed before the decision for surgery is made. If the issues aren't addressed, there is a very real danger that the surgery will be blamed for their problem and they will de-transition. Ideally when you complete surgery, you should be steping into a new problem free life.
There are some conditions that may require life long treatment such a bipolar or schizophrenia however as long as these issues are properly controlled, they shouldn't prevent a transition. How much therapy is required? That somewhat depends on the honesty of the patient and how quickly the issues are resolved. For somebody with serious issues, maybe once a week. For somebody free of issues, possibly a few appointments after the initial diagnosis.