What is the fuel consumption of an aircraft in a holding pattern?
The fuel rate while holding depends on elements such as weight, altitude, speed and aircraft configuration.



Airbus uses the concept of green dot which is the speed with minimum fuel consumption per time unit in clean configuration.

This table shows fuel needs for a A330/A340 at 170 tons and 1,500 ft:


Fuel flow vs. speed. Source

While the green dot speed is the one offering the best fuel rate while holding, it assumes no flap and no slat. This speed may not be practical for certain airports where slats or flaps are advisable due to the rate of turn in the holding pattern.

To provide the necessary flexibility, Airbus publishes the fuel rate for three other combinations of speed/configuration (S speed is the minimum speed for flaps retraction):


Fuel flow vs. weight for 4 configurations. Source

Altitude is also a factor preventing an optimal fuel flow. Example for the green dot speed:


Percentage added when departing from the optimal altitude. Source

The total fuel required to hold can be somehow minimized by not flying at cruise speed when it is known that a hold will be required at some point. Savings can be done by flying at green dot speed to the holding point:


Source



That means determining the fuel flow depends on airline operating procedures and holding pattern design (e.g. altitude), but a maximum can be roughly predicted by looking at the manufacturer published documentation.