Exorcisms may last between several minutes to several hours – usually they take several dozen minutes. Exorcisms are usually conducted several times, until a full freeing of a given person is achieved. In some cases they may be continued over a number of years.

Jesus taught that an evil spirit cast out of a person might try to come back, unless that person takes care to purify their soul of sin: “When an evil spirit comes out of a man, it goes through arid praces seeking rest and does not find it. Then it says, ‘I will return to the house I left.’ When it arrives, it finds the house unoccupied, swept clean and put in order. Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there.” (Matthew 12:43-45) Therefore it is crucial that the freeing be accompanied by a decision to convert. God, during the exorcism, often gives enslaved people a very strong sense of His love and grace, necessary for rejecting evil. Therefore, the personal bond and love for God should be rebuilt through the reading of the Holy Scripture, sacraments, prayer, renouncing sin and penance. A true and honest conversion brings closer to God and results in a lasting removal of demonic influences. 

The effectiveness, and therefore the need to continue an exorcism may depend on many causes:

• the penitent’s fight with the demon, that his their will to convert. 
• the degree of the evil spirit’s actions. Sometimes one blessing is sufficient (e.g. of the possessed places), while at other times multiple prayers are required (a full possession).
• the cause, time and manner of the enslavement – small, or even unborn children might be victims (through a curse or being offered to an evil spirit). 
• the evil spirit’s strength (his place in the hierarchy of hell) and the number of demons that possess the person.
• the exorcist, his piety, charisma, preparation and the methods he applies. 
• God’s Grace. Sometimes the possession or demonic tormenting is impossible to remove – a situation experienced by many saints, who undertook the fight against the evil spirit, protecting others and as a result of their sainthood (Saint Father Pio, Saint Jean Vianney, Blessed Mother Theresa of Calcutta). God allows the actions of the evil spirit and the exorcism does not bring freeing, but it does grant the grace of enduring a trial. The case of Annelise Michel’s possession falls into this category.