Hi weird,

Mr. Google knows. Aloha

I tried Mr. Google, but he wasn't very wise :)

Wierd,
but true.

I had to do a mind-meld but I forced Mr. Google to reveal the following:

http://www.efficientirrigation.com/

http://www.aquaspy.com/default.cfm?id=201

http://news.ufl.edu/2007/11/13/water-sensors/

http://www.cheapvegetablegardener.com/2009/03/how-to-make-cheap-soil-moisture-sensor.html

Hope this helps. IMHO The last site is the way to go with just a few reading over time will educate you about various area need patterns at certain times of year and you can set your controller duration with confidence. Aloha