The C A P ART-DNE is an adjustable timer used in many hydroponic and aeroponic systems. The CAP(Custom Automated Products) timer is something of an industry standard and considered by many to be the best available interval timer.
It has earned a 4 out of 5 star review rating on Amazon which is pretty good but like all electronic stuff occasionally these timers have problems. My personal preference is the CT-1 Short Cycle Timer which has worked flawlessly, at least for me.
C A P ART-DNE Adjustable Hydroponic Interval Timer
The cap art dne offers day, night or both cycles and can be adjusted to a time of 1 second to 40 minutes on and has an off time of 1 minute to 8 hours. This makes it perfect for aeroponic systems that want short off and on times.
Most conventional timers run on 15 minute intervals which may be OK for an ebb and flow system but is far too long a cycle to make them usable for aeroponics.
I run my grows with 1 minute on and 5 minutes off and in my system this works perfectly. But some growers like shorter cycles like 30 seconds on and 2 or 3 minutes off.
There is no hard and fast rule because every system is different. So you need to check your root system and adjust your settings accordingly. But you need a timer like the cap art dne timer that allows precise control down to the second to fine tune your system.