Here's how I work out how often and how long to mist a cage:
Get the hygrometer placed and let the reading settle. Record it.
Mist the cage however long you are doing it now, and record the resulting humidity level. Too high? Too low still? Just right?
Let the cage dry and watch the humidity level fall as it does.
Record how long it takes the cage to get TOO dry, and how low the humidity gets.
Mist again until the humidity rises to your desired level.
This tells you how long between misting cycles you can go and how long to mist again to get back to the level you want to maintain.