Scientific studies have shown that it takes 90 days to change a neural thought pattern that was ingrained in the brain through addictive behaviors.
I was listening to a shiur from Rav Pam Zatz"al about how saying 90 times Mashiv HaRuach makes it "ingrained in our minds"... and he said (in yiddish) "Az Mir Zogt Tzaddik P'amim..." and it hit me. The letter Tzadik is the gematria 90! So if you hit 90 days clean, not only have to made sobriety engrained in your mind - but you're a Tzadik too!