I always use a timer on my drip systems. There are a lot of factors that affect what on/off cycles I wind up using though. The amount of overall
growing medium is probably the biggest factor. The more of it there is, the slower it will dry out (therefor longer times between cycles). Also heat and humidity. As long as the growing medium where the roots are is continually moist (not saturated), they should be fine. Just stick your fingers down into the growing medium 2 or 3 inches before the timer is set to go on to check if it's still moist or not.