If you didn't have a tolerance to Opioids, you would certainly die. When end-of-life euthanasia is administered, usually with some kind of Barbiturate, an anti-emetic is typically administered.
It would be hard to say how quickly you would be dead. I assume that your last moments would probably consist of violent emesis. Methadone is a long-acting Opioid. This is not what one would want. A large injection of Heroin with an oral Benzodiazepine 30 mins prior would be better suited to this task.
If you are really suffering and have no relief in site, then I respect your right to end your life. If this is the case, I would reccoment goint to a "right to die" state like Oregon or Vermont and have a physician oversee the process.
If you are considering suicide due to depression or anxiety, then PM me and we can talk about it. We can also talk about it right here. We are like Planet Fitness, this is a judgement free zone.