How long does Methadone take for a person to feel it being that the person is taking 20mg for the first time?

Being that this person hasn't taken this med for months and has only been on 1-2 Oxycodone 5mg/ (325mg acetaminophen) every six hours for two months. Now starting Methadone regularly with Oxy for breakthrough pain. Some say Methadone peaks at three hours but other say it works just fine in 20-30 min. I took it over two and a half hours ago, along with on Oxy, and am barely feeling about 5% relief just now.