The determining factor is; Are the roots getting enough water.
Arbitrarily set up a time frame.(say two hours) then during the heat of the day (usually early to mid afternoon) watch the plants for wilting. If they wilt before the pump kicks on, figure how long it has been and subtract about 15 minutes.
if the pump kicks on before they wilt add an hour and watch them again. try to pick the worst time of day (for the plants) and set you're timing based on that.

As grizzman stated- there are many factors that go into determining a watering schedule.

If you want a quick and dirty answer- 15 minutes on, 45 minutes off 24/7 should work just fine for almost any top drip system.
If you want a scientific answer- You can watch the plants and if they (or the media they are in) begin to dry out, it's time to water again. Don't be shy to poke a finger down into the media to see if it's just dry on top or if it is dry all the way through.

Whatever you do, don't buy a moisture meter. Your fingers work just as well at telling if the medium is too wet or too dry and they don't cost anything.

RE: "...15 minutes on, 45 minutes off 24/7"

Wouldn't this increase the chances of root rot?

Thanks,

Clueless in Colorado

I doubt that timing is too much water. your medium isn't that absorbent.

Clueless in Co:

I run my hydro pump 16 hrs/day and have never had root rot.

i HAVE A SMALL nft SYSTEM, MADE OUT OF PLASTIC TUBS & NET POTS. hAS BEEN RUNNING ABOUT FIVE MONTHS, 24/7. oNCE THE ROOTS ARE DOWN INTO THE NUTRIET SOLOUTION, IS IT NECESSARY TO KEEP PUMPING ? If so, how often & how long? Thanks, Bill

My aeroponic system runs 24/7 no matter what Im growing. I also have an airstone in each system pumping lots more air through. I guess if you have enough aeration and agitation to introduce oxygen into the system, then doing "cycles" may work. In the least, keep the pump on during the heat of the day to add extra oxygen, and maybe try cycling at night or as the hot sun lowers. During the day when the nutrients are at "max" temp you'd want the pump and aeration going.

Here is a link that might be useful: http://www.TechnologyGarden.net

You've got to keep in mind that different types of hydroponic systems can get away with different things. Running the pump all the time in an aeroponic system won't cause root rot, but doing it in ebb and flow will kill your plants fast.

In drip irrigation you don't want to run the pump full time.

Since you're growing outside there's not hard and fast time to program in. In fact, you'll probably need to adjust it slightly as the seasons change. You want the media to be just about to dry out when the next watering cycle kicks in at the hottest time of the day.

Root rot is only likely to be a problem if your roots are sitting in water or very, very wet media for extended periods of time. Fifteen minutes or so isn't going to do any harm and as soon as the watering cycle ends the excess water will drain away.

Aeration in a drip system is nice, but not generally necessary since the draining of the water draws fresh air down into the medium.