Hello solid7
That's probably the most common question everybody asks at first (including me). But the realistically it all depends on many variables. The real answer is general rule of thumb, it only needs to be on long enough to completely wet the roots, and not off so long that the roots begin to dry out.
With all the variables like system design, type of
growing medium, how much growing medium is used, air temp, water temp, humidity, type of plants, size of plants, evaporation/transpiration rate etc., that a general rule of thumb is the only real answer. Observation is the key to how your plants are doing. Every grower has there own preferences, that's why you see so many different on/off times/cycles. As long as the growing medium remains moist and you don't see any singes of wilting at all, you should be good. But it's in your best interest to try different cycles, observe how it affects your plants and make your own decisions on what you thinks best for them. It wont be long before you'll have a great feel for it (and even have your own preferences).