Methadone builds up in your system the more you take it (you used it 3 days out of 4, so that's different than just taking it once), and individual metabolisms vary greatly, so it's going to be different for every person. Methadone takes a very long time to be completely out of your system, but if a tiny bit is still in your system shouldn't make much difference as far the possibility of a kratom overdose, so you don't need to wait for it to be 100% out of your system, but it is safest to wait until you are not experiencing any effects at all from the methadone any more (and I don't mean just the "high", the respiratory depression can last after the high is over) and for a good portion of it to be out of your body. Kratom is not a very strong opioid, especially in comparison to methadone so if you are no longer feeling any effects from the methadone, you were able to handle 30 mg of methadone without overdosing, and it's been 2 days since you last used methadone, I would say it should be ok to use some kratom, just start with a small amount of course since you can't be certain how it will affect you.
As for whether you would fully feel the kratom, that's a tough one too, as it again depends on your metabolism and tolerance. The methadone should no longer be significantly blocking other opioids by this point, but if you were able to use 30mg of methadone without overdosing you may have a reasonable tolerance to opioids, and your tolerance may have gone up somewhat from using methadone 3 times almost in a row, and kratom may not feel like much in comparison.
Personally I would suggest you take a longer break from opioids so as not to end up physically dependent. I think that is the biggest risk here.
If you want to know how long it takes methadone to be completely, the half-life of methadone is extremely variable and dependent on individual metabolism and single versus multiple dosing. It ranges from 4-190 hours. On average it's probably about 15-60 hours, (at least presumably for people who haven't taken it chronically). So that means half of the drug will be excreted in that time. So for 3 half-lives to pass, meaning that 87% of the drug would be gone, would take most people around 75-180 hrs, or around 3.1-7.5 days. And if you were one of those people for whom the half-life was even longer it could take anywhere up to 24 days for methadone to be 87% eliminated. For it to be 100% out of your body could easily take months, so I doubt you'll be waiting for that to happen before taking more opioids.
