How long is a former heroin addict meant to be on methadone before being completely weaned off?

My dad is on methadone and I am worried that he doesn't actually want to get off it as it is cheap and apparently "helping" him.

o.O

: http://alcoholdrugstreatment.info/how-long-is-a-former-heroin-addict-meant-to-be-on-methadone-before-being-completely-weaned-off