In general, an exorcism is a long and tedious process over months.
Only if it is a full exorcism. Minor rites of exorcism are much shorter and usually only span over 1-2 days.
One of the main reasons for this is that a priest can usually conduct a minor rite of exorcism at their own discretion and typically conducted if a single diabolical entity is identified.
A full rite of exorcism (or the rite of solemn exorcism), which (in the Catholic belief) can only be conducted under the consent of the Church (and typically following consent from the Vatican), usually requires an investigative period by the Church prior to giving consent to conduct the ritual. Then, an exorcism may be conducted over a period of 'sessions', depending on the entity and what its intended target's physical, mental and spiritual position is. A full rite is normally allocated to a senior priest with years of experience in exorcism but may be assisted by a number of other ordained priests. The main reason behind this is that the most extreme possessions normally involve a 'top ranking' demon who holds a legion of lower demons (or minions) under their control and as such, the possession will normally involve more than one diabolical entity.
An example of this was the account of the exorcism of Michael Taylor from Leeds, England in 1974. The conducting priests documented that they 'pulled' over 40 demons from Micheal, all under control by a higher ranking demon. Unfortunately, following this period of exorcism, Micheal went on to kill his wife in a very brutal way before strangling the family poodle. The Church believed that a number of demons remained attached to him after the initial ritual was conducted. Micheal had no recollection of the event when stopped, wandering the streets all bloodied, by the Police following the incident and spent 2 years in a psychiatric hospital before being released.
Where the Catholic Church requires priests to be trained in the ritual of exorcism rites, in principle, every Christian has the power to command demons and drive them out in the name of Christ.
Now,
that's a scary thought...........