There are many variables when it comes to length of treatment. It depends on the type of therapy, ability to trust, and to some degree the diagnosis. I have depression caused by ptsd and years of child abuse. My therapist and I have discussed possibly trying emdr however due to my extreme trust issues and inability to go into details about the abuse it would likely not work for me. Sometimes just bringing up the topic in session can cause a downward spiral. So we take it very slow.