It is recommended to listen to subliminal messages for 4 to 12 weeks, or until you notice a clear shift. From then on, use the recordings when needed to maintain the intended change until it is well-consolidated.
If you are listening to programs that also contain Binaural Beats and/or Isochronic Tones, limit the usage to an hour per day/night. Some of the recordings for sleep aid contain 90 minutes of frequencies to induce sleep stages in the brain. Aside from these recordings, do not exceed an hour of usage per day.