Background Sick leave in patients with a mental disorder is characterized by having a long duration. Studies suggest that the time until a patient on sick leave for a common mental health disorder initiates evaluation and treatment by a healthcare professional is an important factor in the duration of the sick leave. However, in these studies, the intervention was not performed by a mental health specialist. The aim of this study was to find out whether the length of sick leave was associated with the time before initiating psychotherapy, age, time until returning to work after psychotherapy ends, and duration of psychotherapy. In a further analysis, we examined whether the model composed of age, duration of psychotherapy, and time before initiating psychotherapy predicted the length of sick leave. Method The sample consisted of 2,423 participants, 64.1% (n = 1,554) women and 35.9% (n = 869) men, who were on sick leave for anxiety disorders or depressive disorder. Results The total duration of the sick leave of participants diagnosed with depression and anxiety was positively associated with the time before beginning psychotherapy. Time before beginning psychotherapy predicted the length of sick leave when the variables age and duration of psychotherapy were controlled. Conclusion It is necessary to reduce the time until beginning psychotherapy in people on sick leave for common mental disorders.