I think the best way to measure this would be to think of it as a fraction of fuel consumption of a car going 30mph (city speed limit in NYC.)
The scientific way would be to express it as a volume measure, something like 10cc per minute or something (I’m making this up.)
What do you think?
Here is my guess:
A car idling will use 1/5th the amount of fuel used typical car is traveling @ 30mph.
What’s your guess?
What is typical ? It varies with the car, the condition of the car, the elevation and the needs of the driver. Leaving the AC on would be more and a car in need of a tune up, greater still. A large truck may burn as much as a gallon per hour, so just work down from there. In most cars, which I guess would be nearly half that, it may be better to restart if it’s thirty seconds or more…but in Texas on a summer day, it’s not a factor. Either way, I’m not loosing any sleep over it…zzzzzzz. I don’t think you can compare it to a car in motion where there are more factors still to consider like load and terrain.
@dagosa…I couldn’t have said it any better.
1 gallon per hour seems to be at the top end after scouring the internet, but I will use that for my 6 cyl trailblazer. I get 22 mpg hwy, so I’ll round it off to 20 for easy math. That would come out to 3 gallons per hour highway at 60 mph. So 1/3 gal per hour of highway driving seems to be the ratio.
So if you know your highway mpg, you can calculate gallons per hour, divide by 3 and that would be my guess for your car of gallons per hour at idle. Using the heat probably would not affect gallons per hour, but using ac would increase gallons per hour imho 
Just a guess! don’t need Dewy Chetham and Howe coming after me.
It has been my experience that a truck engine will burn between 1 gallon and 1.5 gallons of fuel per hour idling. A Smaller vehicle with a 4cyl I would say .2 to.3 gph I would add .1-.2gpg for each additional 2 cylinders.
Now a prius? When it "Idles"
Not much. The engine will turn on for a bit about once every ten minutes to charge the battery. People use this technique to have a/c while camping. I believe that the estimate is that a tank of gas will last about a week. However, it depends on the temperature you set and the outside temperature, so the specific use will vary.