Unfortunately there is no answer to that question. Ecstasy use, even with extreme moderation can have negitave effects, so the answer is based on a persons willingness to push the envelope.
The general concensus is one a month. But honestly if you did E once a month for the rest of your life I can say almost for sure that it will loose its appeal.
I heard of some recent study said once every 5 months and you will not notice a deline in the high, but that is pretty unrealistic for most people, and might not even be true.
So in conclusion any MDMA use is unhealthy. But for harm redution purposes you should use anywhere from once a month to a couple times a year. And listen to your body. If every time you roll you get the comedown from hell then you should probably cut back.