Answer:
Eating disorders such as bulimia nervosa and anorexia nervosa can take anywhere from 5-10 years or longer for a full recovery. This depends on the patients willingness to get "better" and the support they receive from family, doctors, etc...
Every case is unique and requires different treatment approaches. In the majority of cases, relapses will and do occur often. This does not mean the patient has "fallen off the wagon". Relapses are normal and expected during a recovery period, although if the rituals become habitual to the patient again, they are no longer in true recovery.
The process is slow because the patient has to learn how to readjust to life without their eating disorder. This causes distress in their life and ultimately stress triggers many people's disease. The disease is their coping mechanism. To get fully recovered, one must learn to not use food, restriction, or purging as a coping mechanism and find a healthier alternative instead.