What size is the tank? Vert or horizontal? What is the size of the vent approximately?
I mist heavily once per day with a 2" vent the length of a 20G tank.
If you see condensation on the sides thats a good indication there is sufficient humidity. On my 20G it fogs up heavily for about 2 hours then slowly disappears throughout the day. I usually mist in the morning.
There isn't really a rule of thumb for this... I'd just mist as often as it takes to keep the humidity up and make the inhabitants happy. A good humidity gauge can help you out if you want the keep humidity in certain ranges. Also, you can always cover up some/all of the vent if you'd like to keep the humidity higher with misting less.