It's pretty hard to say because I don't think the halflife of the compound is known. And then also the dose would be a factor, or if you redosed at the end of a session, etc. And who knows how long the halflife of the metabolites are? I think norketamine hangs around for a long time, much longer than ketamine, methoxetamine also really seems to hang around significantly longer than ketamine so normethoxetamine may even be much worse.
If you take methylone first, well I would wait until well after the residual stimulation disappears. Because those residual effects come mostly from the metabolites (such as HMMC but I don't know which ones are worst), that have cardiovascular effects. But still the answer is unknown for the same reasons as with MXE: the halflife does not appear to be known and it depends on the total dose how much time your body needs to excrete it.
The question is also if the metabolites are expected to have more interactions or less.
Also if you ask what is 'safe', do you mean to be on the really 'safe' side or whats probably good enough? If someone says you can take MDMA every 1-2 months without visible harm that is only true in the 'good enough' sense. What is really safe is a lot longer but people don't tell you that.
So I don't know. Maybe 3 days to a week? Depending on a lot of factors.
