There is no law mandating the frequency of follow-up. Doctors who prescribe Suboxone for addiction treatment DO sign a statement promising to keep below the capped number of patients, to have the ability to refer for therapy and to refer if indicated, to keep track of quantities of pills prescribed to prevent diversion, and a few other things-- but the absolute frequency of visits is not one of the things spelled out.
As in other areas of medicine, doctors are guided by the 'standard of care' in their area-- i.e. the way that other competent doctors practice. I go on discussion boards for physicians prescribing Suboxone, or talk to doctors at meetings, to see how other docs are practicing in regards to Suboxone. In my area, some doctors (usually those in health 'systems') require group therapy or outpatient treatment for all patients, at least initially... I do not do that, but rather try to fit the treatment to the individual patient. Patients who do well-- those who never run out early, who don't test positive for other drugs, who are doing well in life (finding or advancing in jobs, doing well in school and relationships, etc) are gradually worked down to a less frequent schedule of visits; the frequency is increased if a patient relapses or is doing poorly in general.
In my practice, if patients have been stable for over a year, I see them every three months. This assumes that I have a good relationship with them-- that I am confident that they will let me know right away if they have problems. During the first year the frequency varies from weekly (or even daily in one case) to every two months in patients who are doing well.
I am not in a position to tell other doctors how to practice-- not even close to that position, actually. But my OPINION is that seeing stable patients monthly is a bit much.