If you are on heroin for three months how much methadone do you need to not go through withdrawal?

Methadone will not cure your heroin addiction. It will only keep you from going through withdrawal if you continue taking it. Once you stop taking the methadone, you will start going into withdrawal.

Methadone, as well as Suboxone, are opiate antagonists. They feed your physical dependence on opiates, and they block an opiate high, so even if you relapse and start using heroin again, you won't get high on it. Heroin, Suboxone, methadone, opium, hydrocodone, hydromorphone, oxycodone, codeine, etc...these are all opiate drugs (made from the opium poppy). Methadone (as well as Suboxone) is just an opiate that won't get you high. It helps you quit getting high, but it does not cure or get rid of your physical dependence on opiates.

When you are ready to stop taking Suboxone, you wean yourself off of it very slowly, usually over a period of several months. I would assume the same is true of methadone, but I have never taken methadone, so I don't know too much about it.