The time: January 27th, 2017, 7:00 AM local. The place: New York City and environs.
Jormungandr, the Snakebot of Doom, has just finished steamrolling New York City into something more closely resembling a gravel driveway.
Now it has returned to the vicinity of lower Manhattan and is busy extracting any iron or steel that it can find in the rubble in order to convert it into steel railgun ammunition.
It is currently firing the six large railguns in its tail, each shot containing about 6.8 metric tons of steel, at a combined rate of 4 rounds per second. If it has to use its small railguns for self defence, they won't add significantly to the rate at which steel is being consumed.
Jormungandr's mouth is currently ingesting any iron or steel it can find in the rubble in order to resupply its ammunition bunkerage. It is easily able to resupply its ammunition faster than it can expend it provided sufficient steel is nearby.
How long would the steel found in New York City and environs be able to supply Jormungandr's appetite before it has to move on to another source of steel?
Consider this to be the real world, with the exception that over the evening of the 26th until the morning of the 27th, New York has been experiencing severe blizzard conditions, leading to closure of ports, airports and roads.
At the time in question, neither roads, ports or airports have yet reopened.
The blizzard will mean that there are more ships in the ports adjacent to NYC than usual, and that cars cannot readily leave.
By New York City and environs, I mean the area with this definition: https://drive.google.com/open?id=1piz-lZJ_ti3EAnEH7b_LNgIguiQ&usp=sharing
Please account for the quantities of any man-made iron or steel within the area, no matter how large, including any vehicles, shipping, building structural members, railway track, rolling stock and tunnel shoring or lining, with a reasonable justification. The exact amount of steel in each building need not be accounted for.
Small objects such as nails or bolts, unless part of a larger steel structure, may be discounted, as Jormungandr will not bother with anything so trivial. This will rule out its being interested in scavenging from wooden-framed dwellings, but steel framed dwellings are another matter.
The world in this question starts out pretty much as the world with which we are familiar. However, the question is, How long can Jormungandr stay here before it needs to move on to another source of steel, and the world being built is one where Jormungandr stayed for x time, firing continuously, then had to leave to find more steel. That world may or may not resemble our own so much, depending upon the magnitude of x.
I want to know X. Can it stay for Hours? Days? Weeks? Months? Years? How many of them?
If Jormungandr stays for 'hours', then the world will be considerably different than if it stays for 'weeks'. Hours means that all that many of the survivors of NYC have to do is sit tight and wait for relief efforts to come to them. Weeks means that the survivors have to become refugees and travel to somewhere where they can get the necessities of life, since Jormungandr isn't going to leave much in the way of the necessities of life for the survivors to use, and how many relief agencies are going to risk coming close to a thing that destroyed US armed forces sent against it so easily?
The longer Jormungandr stays put, the more time humans will have to try to come up with unconventional means to try to destroy it. The longer it stays put, the more likely that humans will discover that the birds have been bugged before it moves on. The longer it stays put, the more likely it is that some idiot may try a massed submarine ICBM launch against it.
So, while, yes, this is a question about a story set in a world, the world-building aspect of the question is that the answer affects how the world changes. Please don't think that just because the question is basically "how much steel is there in NYC, and then divide by 27.2 metric tons per second" that there aren't other implications that will greatly affect that initial, known, world.