The length of time it takes for Eye Movement Desensitization and Reprocessing (EMDR) therapy to work can vary depending on a number of factors, including the severity and complexity of the traumatic memories and emotions being addressed, as well as the individual's overall mental health and level of engagement with the therapy.

EMDR therapy typically involves several phases, including history taking, preparation, assessment, desensitization, and closure. The length of each phase can vary depending on the individual's needs and progress. On average, the assessment and preparation phase can take a few sessions, however, the desensitization phase, where the eye movement or other bilateral stimulation is used, can take several sessions to complete.

It's important to note that EMDR therapy is not a "quick fix" and it can take several sessions before an individual begins to notice improvements. However, some individuals may experience a significant reduction in symptoms after just a few sessions, while others may require more time and sessions to fully process their traumatic memories and emotions.

It's also important to note that the length of time EMDR therapy takes to work is not the only important factor to consider. The outcome and effectiveness of therapy is a complex and multifaceted topic, and the quality of the therapeutic relationship, the therapist's experience, and the clients engagement with the therapy are also important factors to consider in addition to the duration of the therapy.