For a Panther I would do three mistings a day. Personally I wouldn't buy the misting system, I'd buy the RainDome system... but that is just me.
As for the mist or rain durations and frequencies. I would stick to three operations per day with a panther. I would do the first about 1-2 hours after their heat light is on, somewhere around half their day time and then a third about 2 hours before lights out.
As for times;
If misting, #1: 5-10 min, #2: 3-5 min, and #3: 4-6 minutes.
If RainDome, #1: 8-12 min, #2: 4-8 min, and #3: 6-10 minutes.
This is all of course if you have the drainage for that volume of water. I sell drain trays and custom order trays at
AquaZamp.com
The reason I suggest a RainDome over misting is because the mist tends to scare off the chameleon and they hide away from the mist. If they are hiding they aren't getting wet. This doesn't allow them to soak their skin to have beter shedding and they don't get a chance to clean their eyes out. Sometimes they get over their fear of the spray nozzle. But with the RainDome is is way more natural for them to have large water droplets falling from above. I have never seen an animal react so instinctually as when they experience water falling in the cage and move over to it to drink and bathe.
Sometimes the best mist/rain setups are just found by watching the chameleon's behaviors.