How long does it take a person to be detoxed from methadone?

How long does it take a person to be detoxed from methadone? He has been on the drug for five years and is taking 100mg a day.

The usual rule of thumb in detoxifying someone who has been maintained on methadone is no more rapidly than 10mg decrease twice each week. By this rule, a patient on 100mg would be detoxed in about 5 weeks. BUT the other rule of thumb is that the slower the better. The real problem is that at around 30-40 mg per day, the craving for opioid drugs very commonly recurs, and that craving leads to significant distress for most patients - especially those who have done well during treatment. So . . . if someone WANTS to detoxify, despite the fact that there is always a risk of relapse associated with detox in even the most successful patients, then the slower the better, and both patient and doctor/staff should be ready at any time to reverse or delay the process if craving does recur. We know that there's always a likelihood of relapse after ANY treatment, methadone or drug free. That being the case, if the patient has no side effects from the methadone, and very few patients do, then the more conservative and the safer course is to continue the treatment.
The amount of time the individual has been receiving methadone treatment is not a critical factor.