well, you don't know what are OP's preferences. if all he's worried about is brain damage, *never* doing MD is ideal. that is because nowadays, many if not most researchers believe MDMA and MDA are neurotoxic. even if you drop just once every three months, you're probably still causing damage to your brain.
if, instead, he wants to space his use so he can enjoy the "full" experience every time, i don't think you can give a suggestion of "once per month max, preferably only once every three months." it is my understanding that we simply don't know yet how long it takes until your serotonin levels are restored. from
what i've read (i'm just a layperson, though), it is actually likely that it takes *less* than a month for SERT density to recover.