For some of us (or most, not sure exactly) even 30 days is too much to think about.
The 90 day goal is a fancy scientific thing, changing your brain patterns, blah blah blah. It is what it is, but for me, it only works when it is in the background.
Every day is an accomplishment. Besides for the obvious benefit of being clean, the more we train ourselves not to chase after physical pleasure as a life mission, even if we sometimes fall, with every "no" we begin to rewire ourselves.
One day at a time?
Do we ever overcome our Yetzer Harah completely?