I used a good deal of water. It takes a higher wattage to heat a larger amount of water and keep it heated. That is why certain wattages are recommended for certain size fish tanks. Here is a chart that actually may help:
http://freshaquarium.about.com/cs/ti...heatersize.htm
Sure it's referring to aquariums but it's the same concept. Make sure you get the incubator set up a week or so in advance of any expected eggs. You want time to stabilize the temp and get the egg box temp right.
Good luck!