With self watering containers, they can not be overwatered, since they have an drainage hole at the top level of where you want the level to be. I think I was watering for 20 minutes at during the warmest part of the summer and while I was away. Earlier and later, I watered every other day. All part can be bought at Lowes, except the drippers and replacement connectors, which I ordered from Greenhouse Megastore. I think I ordered the 36 inch ones so they could go from the tubing on the ground and up to the watering pipe and into that a ways so it was secure.
Excellent description of self watering container construction is
http://josho.com/gardening.htm
Ray has a slightly different one in the workbench section of this site.