If you have received warning signs, then the sooner you quit, the better. The longer you let it go on the worse things will become and the harder it will be just to let it go. Mdma use was never much of a problem for me, but rather cannabis abuse - I should have stopped years ago, and it's rather scary how time will fly by as my attachment becomes stronger and stronger and I lose myself more and more to a pointless obsession. Mdma, as something that cannot be used frequently, should be relatively easy to quit compared to something like weed that becomes a morning-til-night lifestyle. That's not to say someone can't roll once a month and obsess over it every day in between... I think when you start obsessing over a drug or realize it's had a negative impact on sober you, then it's time to stop.
Just look in the mirror - see what you've become, how you've changed, how it's affected you. If it's negative, there's never a better time to stop than right now. For myself Mdma was entirely positive and life changing for the better - I just decided to quit while I was ahead, because I need to learn how to be happy and open to others all the time, not just when I'm rolling, and further use, going back, just doesn't help me accomplish this at all.