My Miata has 550 cc/min injectors, running sequential, and datalogs show that commanded injector pulse widths at idle are about 1.7 ms, so with an injector dead time of 1.0 ms that means 0.7 ms of fuel per squirt. Hopefully I won't screw up the math too badly.

injector flow rate of 550 cc/min = 9.167 cc/sec
x .0007 sec = .0064 cc of fuel per cylinder per revolution
x 4 cylinders x 850 rpm = 21.8 cc of fuel per minute
x 60 min in an hour = 1309 cc/hr
1309 cc is 0.35 gallons burned per hour at idle.

If I go by the datalogged injector duty cycle % instead of injector pulse widths I come up with 1584 cc/hr = 0.42 gallons/hr. Not sure which is closer to reality, but 0.35 to 0.42 is probably a good ballpark range.

This is a 1.8L engine idling at 850 rpm and 15:1 AFR. It's also using mid-late 80s RX-7 injectors which have crappy spray patterns compared to modern tech, though the Megasquirt does idle them pretty well.