You can in some instances over mist. The cage should have a chance to dry out completely between each misting. You also want to avoid puddles of standing water in the bottom of the cage or on top of the plants. Puddles of water are a breeding ground for bacteria. Your feeder insects will also drown in these puddles allowing for even more bacteria to grow. If the cage is constantly wet, mildew can form. 3 good misting sessions a day usually does the trick with mine, misting around 10 minutes each time.
I actually try to avoid directly misting my chams the day that they are shedding. It seems to make the shedding skin stick to them a bit more, although that may just be in my head
My Male Veil seems to feel more comfortable when I put him underneath the kitchen faucet sink for a wam shower when he is doing major shedding. The water seems to soften his skin and acts like a natural mineral oil lubricant so that he can shed more easily. Also, I notice that he gets a little itchy on parts of his body where there is major shed and it relieves his itch when he is moistened with water.