The federal regulations for heroin-assisted treatment state that it has to be six months, but for oral opioids such as methadone or buprenorphine, there's no such period, so it's up to the discretion of the prescribing physician.
You're absolutely right. There's time to build up a relationship between nurses and doctors and the patients, and then, when the patient is sufficiently stable, we provide them with take home. At the start of treatment, it's always supervised.