Believe it or not there are a lot of things to consider. How dry is your climate? Do you have a lot of live plants? What kind of substrate are you using? Do you have a water feature in the tank? Do you have the lid partially covered? All these things would increase the humidity in the tank. I would start off with 30 seconds in the morning and evening then increase or decrease it from there depending on where you need it to be. Use a good quality digital gauge to get an accurate reading. I have a Zoo Med Hygro-therm monitor and keep my humidity right where I set it at.