Enough to keep it damp. And that depends on local weather conditions

The material to be composted needs to be moist, damp may be too wet. I have found that if the material I put in is moist at that time I seldom had to add more water as the process moved along, but the best indication is smell. Does the material smell like good rich earth? It has enough moisture, Does the material have a putrid odor? There is too much moisture.

As an example ... In Phoenix AZ I have a section of drip emitter line in every wire mesh compost bin, running for about an hour every three days. It delivers about 10 gallons of water.

It's enough to keep the center moist, and the outer layer stays dry. I put it in the bin when I start filling it and keep lifting it up to keep it on top of the material.

I don't turn the bins.

In our arid environment I put about 15 gallons of water per week in a compost bin that is about 18 cubic feet. I do have drip emitters in some of the compost containers. They are on automatic timers for the plants, and really help to insure that the compost stays moist. In bins where there are lots of worms, I need to add even more water. Unless I turn the pile my outside is almost always quite dry: our humidity during the summer is often in single digits.
Renais

This post was edited by Renais1 on Tue, Aug 19, 14 at 13:47

I'm a lazy composter with usually a cold bin. I don't turn often due to back issues. I usually water my vege garden 2-3x per week, so I give my bins a sprinkle then. I let the winter rains (what rains?!?) take care of the winter watering.
I usually end up with 2 batches a year, spring and fall.
I HAVE tried some intensive composting with rabbit poop and coffee grounds from a coffee place and I admit that it came out much better, but again, I'm lazy! ;) Nancy

This is very helpful, and probably others have this same question. Thanks to all of you for your input! Much appreciated!