Like Carlos said they vary by brand and the tank ventilation will affect how long it needs to be on. Start out by misting doe 15 - 30 sec every 4 hours and see hoes that works for your tank.

Keep an eye out for how wet the substrate is the next time you mist. If it is still soaking wet you will run the risk of swamping out the soil to maintain humidity.

To keep the humidity up you can reduce the ventilation, add Long fiber sphagnum moss to the top of the soil, and look into LED or Florescent lights.