Actually about 3 to 5 seconds per minute for low presssure. About 2 seconds per minute for medium pressure and about 1 second per minute foe high presuure or air atomized is usually enough and that is using low capacity mistesr sucs as 1 gallon per hour. Most of the little cheapy misters (they typical green., blue or red ones made for spagetti tubing are as much as 10 gallons per hour, Gross over kill). The easiest way to handle it and also get the best life span out of your pump is to actaualy run the pump 24 hours per day. Install a T fitting and an electric solenoid valve controlled by a timer. when the solenoid is open the water flows back into the reservoir. When the solemnoid is closed then the T is closed so the water flows through the misters. Nearly all aquarium pumps and fountain pumps are made to run 24/7 not intermittant. They last longer when not turned on and off frequently. In actually the rotts should be inveloped in a mist od droplets not sprayed. when that happens they develop hair roots not big fat tap roots and feeder roots. The system actually works better if the roots never grow down into the water. Some people install shelves made of the plastic grates used with flourescent lights and cover that with a layer of silk screen cloth. The plantds grow much beteer when the roots just stay moist not wet and definitely not in the water. They plants will grow with roots in thw water but then temperatures of the water must be lower and oxygen levels higher. Keep the roots jsu moist in air and those are not big issues anymore. Plus the internodal spacing is always closer when the plants have lots of hair roots. There fore qulity and quanity increases as y the palnts or more compact so the lighting is more intense. It is sorta like, if you can fully bud out a plant 18 inches tall in 6 weeks that produces the same amount of bud but fat and tight or more than a 36" plant in 10 weeks that produces long fluffy stemmy buds, which would you perfer to grow? Especially if the plants started out as identical clones.