Some research I've done suggests that a small idling engine should consume about 0.2 - 0.3 gal/h, which is about 0.8 - 1.1 L/h. However, the following calculations I've done are off by a mile.
At idle, engines typically operate in a rich-running condition, which implies a best-case air-fuel ratio of roughly 14.7:1 (i.e. 1 L~fuel~ / 14.7 L~air~).
According to this article, engines typically have a volumetric efficiency of 15% at idle. This means the volume of fresh air drawn into each cylinder is about 15% of the cylinder's volume.
A 4-stroke engine sucks air into all cylinders in two revolutions.
Using this information, I've calculated the idle fuel consumption for a 1.1L engine at 800 RPM as follows:
rate = (800 rev / min) * (15% * 1.1 L~air~ / 2 rev) * (1 L~fuel~ / 14.7 L~air~)
This comes out to roughly 4.5 L~fuel~ / min, or 269 L~fuel~ / h. What gives?
Anecdotal evidence, I had a 2008 Mazda 2.3l and a device that plugged into the diagnostic port called a Dashhawk. It would measure engine load, and it said the load at idle was 20%, which surprised me.
It's 30% for me (50% with the AC on)