An inpatient program (Inpatient Rehab) requires the patient to live at a facility 24 hours a day. In-patient residential treatment for drug and alcohol abuse is common in the U.S. Most American programs follow a traditional 28-day program length. The length is based solely upon providers’ experience in the 1940s that clients needed about one week to get over the physical changes, another week to understand the program, and another week or two to become stable. Seventy to 80 percent of American residential alcohol treatment programs provide 12-step therapy.
Source: wikipedia.org, psychcentral.com