How difficult will it be to come off of 40 mg's of methadone a day?

Very, if you do it: cold turkey. The level of opioid dependency with methadone, as with other opioids, is highly variable. Some people have very good therapeutic levels at 40 mg and have severe withdrawal that can last for weeks and sometimes even months if done cold turkey. Others have relatively milder cases. If someone is on it for months, then at least 1-2 months to taper it is reasonable.