One of the members of our forum who goes by the name "Jack", has been struggling for 38 years. Thank G-d, Jack is already past 60 days clean with the help of all the wonderful people who offer support on the forum.
Jack's time-line can be viewed here, and his thread on the forum can be viewed here.
Jack was on the "Jewish Healing Group" phone conference once in the beginning of Elul, when the therapist on the line mentioned a recent study that found that it takes 90 days for a person to change the neuron paths in the brain, and thereby change an ingrained thought pattern. The therapist claimed that the study found, that if a person was able to refrain for 90 days from a certain type of behavior they had become accustomed to, it would become much easier afterwards to continue.
At that moment, on the phone-line, Jack decided to start his journey and aim for 90 days. Later, Jack pointed out to me something beautiful. As we know, all wisdom can be found in the Torah. The 7th of Cheshvan, we start asking for rain by saying "Ve'sein Tal Umatar". The Halacha is, that if one is not sure if he said it or not, he must repeat the Shmoneh Esrei. However after 30 days, one no longer needs to repeat Shmoneh Esrei when in doubt, because we assume that his mind has already gotten used to saying it.
Get this. 30 days is 90 Shmoneh Esreis! Chaza"l knew that it takes 90 times of doing something to get the mind used to it!