It depends, frogs like it over 80% humidity...if it gets lower, mist a little.
For plants, most of them like to be able to dry out a little between mistings.
For accurate humidity readings, you really need to have a digital meter, the analog ones don't give very accurate readings above 70%.
The tanks I have with 100% glass tops only get misted when I want to clean the tank up...the plants usually keep the humidity plenty high. The ones with more screen ventilation need misting more often.