Over the past 4 years I have read thousands of posts on this and other forums. It's a rare recovering addict who got and stayed that way due to rehab. The most common characteristics of those in long term recovery seem to be two-fold. They take personal responsibility for their recovery and they have reached a point where sobriety is necessary to live. Many, not all, benefit from some sort of peer group support system, AA, NA SR or other programs.
One of my favorite posters once said, " I gave up so little ( drugs) for so much (my life)".
Most rehabs offer 28 day programs because that is usually the maximum covered by insurance. 28 +/- days in rehab is not a magic bullet that cures addiction. Best case, it can teach a highly motivated addict some of the tools of recovery. The rest is up to the addicted person. The really hard part begins once they leave rehab.
My daughter has always been the sort of child and young adult who is highly motivated for about 48 hours. As soon as she realizes achieving anything requires effort on her part, she loses interest. Then she's bored and prone to blaming lousy teachers, therapists, Mds, this or that. When that does not work, she does the "why bother, I'll only fail" routine.
On the cusp of turning 24, she is just starting to grasp the connection between cause and effect- that without effort on her part, nothing changes.
It's a cognitive maturity thing.