Qutting Oxycodone using methadone?

My girlfriend is an oxycodone addict, she recently started going to a methadone clinic to get off of it. Her only reason was that oxy was too expensive (i.e. I don't think she is serious about quitting, she just want a constant high). Does anyone know the length of treatment with methadone and how successful it is - i cant see her anymore if she will just be hooked on methadone for the rest of her life.