CBT, and some other models of treatment, advocate for beginning therapy with predetermined time limits. For example, manualized CBT for Eating Disorders involves 20 sessions for most clients, and 40 for markedly underweight clients. The authors of the manual discourage extending treatment except to compensate for disruptions in treatment due to life crises, etc., or in cases where there are symptoms that "continue to interfere significantly with the patient's functioning and are unlikely to resolve on their own." That means that treatment may end even in the presence of continued symptoms, as long as the client is making progress.
There are a few reasons given for this approach.
1) Having a set time-limit from the beginning creates and maintains momentum throughout treatment, by putting pressure on both therapist and client to make the most of the time they have.
2) Ending treatment before symptoms have completely abated, assuming that the symptoms will continue to improve naturally without further intervention, based on the treatment already provided, serves to foster client self-confidence, and reduce dependence on therapy or therapist.
3) Scheduled endings are more likely to be planned rather than unexpected, which allows necessary time to be spent consolidating gains and engaging in relapse prevention.
Of course, advocating a time-limit requires determining how many sessions over what period of time is most efficacious for various client groups with various presenting problems. It also does not address the possibility of multiple comorbid conditions that influence each other and cannot be separated to address sequentially. There are probably other potential logistical difficulties as well.
I find myself somewhat dubious of this sort of predetermined time limit, but can see why it makes sense to some theorists and clinicians; I just have not had enough experience with time-limited formats to make an educated comparison. Have you had experience with both time-limited and ongoing treatment formats? If so, what do you see as the benefits and drawbacks of each?