Methadone transition. Of course the risk is that with longer-acting opiates (Methadone being one) there will be withdrawal precipitated with the transition to suboxone. It has an opiate antagonist in it - which makes the idea of abusing opiates much more unattractive (in addition they will provide no euphoria). Typically patients are encouraged to be in full withdrawal before switching over to the suboxone, ? 3 days.
Depends. You have to wait until you are in significant withdrawal, maybe a day or two depending on dose and time of use of methadone. A scale called cows scale can help guide when the objective signs of withdrawal are sufficient for suboxone. The risk with taking suboxonw before withdrawal is inducing a terrible, instant withdrawal as Suboxone kicks Methadone out of the receptors instantly.
3-4 days. Because Methadone is long acting and Suboxone requires that a person be in moderate withdrawal it takes longer. Taking Suboxone sooner risks what is called "precipitated withdrawal" this happens because the active ingredient Buprenorphine only partially stimulates the opiate receptor, if to much Methadone is still there it "kicks it off". Once in withdrawal however it occupies an empty receptor.
Switching. Why switch? Both are addictive and have withdrawal symptoms. Methadone is longer acting, and Suboxone has less withdrawal which is why it is used for all opioid withdrawal, including addition to methadone. The object is to get off comfortably with tapering doses, till the drug is out of your system. That is the goal, not switching.
At least 24hrs... Decease dose of Methadone slowly to 30mg daily and then when you are having fairly obvious opioid withdrawal symptoms about 24hrs or more after stopping Methadone start Suboxone 2mg - 4mg dose. If this is attempted earlier and the Methadone is still not fully detached from the opioid receptors Suboxone will precipitate pretty bad opioid withdrawal symptoms.
Not Here. This is a complicated question which requires the input from an a physician experienced in such things. There is no easy answer. It can be anywhere from 48 hrs to 1 week, depending on many things. Do not try it youself.